@Ithuriel: Thanks for taking the time to write such a detailed post here, and of course, for trying your hand at the high-end difficulty of this mod. A couple notes before strategy discussion:

1) Without looking at the files directly, I would say off the top of my head that the numbers you see are correct. The animated armors are VERY strong when addressed on-level, and they have a wide array of immunities.
2) Playing this mod with the modded tactician vitality makes most any encounter very complex. Every monster that before you could blitz with relative ease now will have at least a couple turns to retaliate, so by default their abilities are more dangerous because you will actually see them. This requires you to adjust your play in certain ways, and if these adjustments make the experience less fun for you, there is no shame in converting this mod to run with the base game's Tactician vitality bonus.
3) The majority of Cyseal's encounters do not block progression in the game. You can skip the ones that your loadout has trouble with entirely or return later with more levels and stronger gear. For certain party compositions, this may be necessary (I did it occasionally in testing.)

Concerning general strategy:
1) "Classes" are a little bit of a misnomer in D:OS. The starting loadouts exist to help players get into the game, but they do not really point the way to a viable endgame. I only look at the starting loadout at character creation to give me starting gear, (if I need at shield, wand, etc.) after that I build freely with certain concepts in mind--this is the engrossing and intimidating thing about builds here: you have the freedom to do basically anything.
2) With regards to damage output: your rogue is dealing a lot of damage due to autocrits on backstab, I take it? Overcharge can also crit and has a decent chance due to its number of projectiles. In a nutshell, to do the damage necessary to address the HP pools you are seeing you need to get as much crit as you can manage on abilities that can crit and then increase damage from the other end of the equation by lowering resistances. Generally physical attacks will try to 100% crit while elemental damage comes through spell combinations that potentially create double or triple damage through negative resistance. In some of the other discussions concerning builds I go into this in detail--check them out if you are interested.
3) Along with damage your defenses are very important. Your tank needs to be VERY tough. This can mean using consumables and buffs to increase resistances for certain encounters, or even gaining status immunities on gear that you swap out, but generally it just means that you need HP, armor, and block chance. Shield spec is extremely powerful and will save you in many cases by canceling entire attacks. Armor is critically important as well--prioritize this stat heavily. A skill like Fortify appears weak, but because of how damage reduction is calculated with exponential increases or decay outside of the level-specific "expected" value, even small armor bonuses can push you into a lot of bonus DR.
4) Someone in the group should be able to heal. Healing allows you to make mistakes and recover, or simply absorb hits. Without healing of some sort, many encounters will probably destroy you via attrition alone.
5) Getting even one or two summons out can take the edge off an encounter, not because the summon will deal much damage, but because they can tax enemy resources. Every attack aimed at a summon is a ton of DR for your party.
6) Having a character that crafts and uses grenades is game-changing at lower levels. This will diversify your damage types to play to enemy weaknesses while giving you AOE where you might not have any. Also, the fact that the grenade talent also gives you crit makes it useful for attackers, so you do not lose much build efficiency.
7) Think about controlling monsters. Having at least one character that reduces grit and/or willpower is a must. You will need to essentially "shut off" monsters from time to time to slow down combat and recover from bad situations, or simply to deal with being outnumbered. This will likely not work if the monsters have high saves, so a certain amount of preparation is often necessary to weaken the monsters and make statuses stick.
8) Hybrid builds (split main stat STR/DEX, STR/INT, etc.) are some of my favorites (I have advocated heavily for them in the past,) and are often very powerful, but they require good gear and higher levels to reach their appropriate degree of power. I see your archer and tank are hybrid. At low levels this can be dangerous because the split focus has a way of making your characters able to do two mediocre things rather than one powerful thing. Think of your party as a single character functioning as a unit and consider each character’s role in the unit. To start it is usually a good idea to have one or two characters that deal the damage and focus on that, then a character that performs support functions (healing, buff, debuff, etc.,) and character that tries to absorb damage for the group.

Strategy here gets extremely complicated and nuanced, and I could fill many pages with considerations to make, but I hope that these overarching notes help you get past the wall you are up against and increase your overall enjoyment of the mod.

Hey, I'll give a more in-depth answer later but I had a few quick questions/clarifications. Firstly, I'm TOTALLY aware of the lack of the class-based system in Divinity; the fact that I have a Wayfarer is honestly a complete coincidence (I wanted to try and experiment with some of the new "gold" cross-skills, and ranger/geomancer had a lot of crossovers).

I'm also fully aware of how the armor system works- my archer was casting Mass Fortify basically every fight. Honestly, I have a bit of a hard time actually using my consumables enough... I tend to really want to conserve them. It's hard for me to figure out how to use the game mechanics without exploiting them- like is it reasonable for me to sneak the rogue behind the enemy every time before I initiate combat? Etc.

I had a healer- the witchcraft/hydro mage. However, he only actually had a single ranged heal available, which made things kind of tricky... the tank had Cure Wounds and the archer had First Aid towards the end, but that was it.

Yup. Summons are good. I was summoning pretty heavily- my archer could summon a spider, and the mage summoned the small skeleton all the time. The spider felt useless, though... almost all the earlygame fights are against undead, who are immune to the spider's poison damage and resistant to piercing. On the same note, this might be why my archer felt totally useless? Most of his spells were poison damage, and bows deal piercing. The mage outdamaged him so heavily that it was almost hard to justify healing at times, because of the amount of the party's damage that would be lost.

Reducing grit/willpower sounds like a lovely idea, man. Except at that level, there isn't a single ability that does so that I have access to :(

You might have a point about hybrid builds, though. My attitude is usually that I need to make sure that I have every school covered in the party, otherwise i might miss out on important capabilities- but perhaps this isn't practical? I'm about to try starting a new run, dropping the archer in exchange for a second mage and running a pure rogue. The archer, frankly, just feel essentially useless; the damage he deals simply isn't meaningful in the slightest. The bloody skeleton summon significantly outdamaged him :'(

...welp, i guess I gave the in-depth answer anyway XD

...although ♥♥♥♥♥♥♥ I'm having to retry the opening fight for like the fifth time now. I started with a hydrosophist/witch (wands) and a rogue, but one of them keeps getting focused down by the three archers.

As one brief example that I'm curious about- in the fight with Ragequin at the end of the tutorial dungeon, one of the skeleton archers has a knock-down arrow. Why? You probably only have two characters at that point; if whoever the archer shoots the knockdown arrrow at fails the resist- at around 25% chance to resist- you instantly lose the fight. How is this fun/how does it contribute to that fight?

I'm not sure if I would call that particular example fun, its more of a difficulty notch. The mod does not pull any punches, so what you find is that at the hardest difficulty it is hard right out of the gate, by design. Although even this can be mitigated heavily. Opportune summon placing, or flat out CC generally stomps this encounter (which was not directly modified, by the way--difficulty is just coming from the overall tweaks.) If you really have a hard time getting out of the gate and just want to get on to the good stuff I strongly suggest that you make characters with the inquisitor starting loadout and then use rain for disgusting CC power. Also, do not be afraid to use your molotovs--they are on your characters at start now to make it so weaker starting loadouts have a tool to get through (this obviously works wonders with Creeping Ooze, especially on the fisrt set of skeletons.) Again the talent that allows you to toss grenades exactly where you want them is very helpful for this. And if I remember correctly it is also possible to have Overcharge out of the gate (been a while since I played,) which is a very good way to get past the first couple fights.

It definitely is possible to have Overcharge, my first run started with that; however, the way I wanted to build my party in order to not waste companion points meant that Jahan was going to be my staff-wielder. Also, I'm a bit reluctant to take Pinpoint since I'm worried that it'll get phased out pretty rapidly, if that makes sense?

Also, one quick question- any thoughts on how to allocate stats as you level up? Mostly I'm just struggling to figure out how much I should be putting into Constitution, really. I know that it's important to get enough of your primary stats to avoid penalties/get CDR, and get speed.

I usually run a squishy party until I find myself reloading too often due to enemy damage output--there are various break points for this based on zone and what gear I have, so I can't really be much more specific, you'll feel it when it happens, either way. Of course, this assumes running a very CC-heavy strategy--which makes you "tough" because you simply do not take much damage. The tougher you are the more mistakes you can make, so HP is often good for progression, and I have often seen reports of people using characters with outrageous HP pools (10K in endgame!) CON is important on the tank character more so than the rest of the party, but this only really shines if you have good armor and resistances. However, at low levels, resistances are hard to come by, so CON can be very, very strong below level 8ish if you are encountering a lot of ele damage. Also, I very often run Water of Life since it allows me to "buy' CON on all my characters while only making an investment on one character. Encourage is similar in this regard. I also often like to get CON on gear rather than in my stats because I generally try to reach appropriate effectiveness thresholds in my skills first (very important on CC.) This being said, do not super-max your stats that determine skill effectiveness since they contribute little once you hit the levels recommended for skills. A perfect example of this is when people wonder why their 25 INT mage deals no damage. The resources required to "buy" the INT simply do not yield enough damage compared to something like more Astrologer's Gaze or points in Alchemist. The same thought applies to Rangers with too much DEX. Basically, the guiding thought behind all of these considerations is what investment equates to the most HP for my party. Will I get more toughness from making the tankiest tank, or will I get more from running super high damage and crazy high initiative so that I kill the enemies before they can attack, or do I play the control game. All of these strategies work, so it sort of comes down to what style do you like the most and feel the most comfortable with.

Honestly... I have a hard time figuring out how to make most of that work :\ You said a very CC-heavy strat- but when you're in a fight against 8 enemies, most of whom have 3+ Grit and Willpower, it's pretty tricky to get many of them CC'd :\ One cryomancer alone, who wasn't CC'd, killed the rogue from full to nothing without me being able to respond at all. Water of Life is definitely a great skill, but I don't have anything like the requirements to cast it at this point in the game- and even if I did, it takes a full turn from my mage. The rogue has 318 health at level 7 in the fight against the Baron of Bones; the cryomancer did 238 damage in a single round, and got lucky on initiative so acted again before any of my party members. I've reloaded the fight like six times now, and that's happened every time I've made it past the first turn... if I don't succeed the opening charm on the court minstrel he pops fear and I instantly lose the fight- generally 2 people get feared, the cryomancer freezes one, and Lady Annah freezes the other. Then the party dies without me getting to act.

Plus, honestly, there's quite a few fights where my tank gets hit by Crushing Fist and knocked down for 2 turns round 1- and I'm pretty much fighting a 3-man fight at that point... it feels like enemies are absurdly more resistant to CC compared to me, to the point where frequent reloading is often necessary.

When you are outnumbered, terrain use and engaging properly often become very important (basically they will win the fight for you.) As far as the engage is concerned, having high initiative on at least one character is very, very helpful. Meanwhile, making an "advance to the rear" will often save a fight that is going sour, or keep one from going sour. Using distance and cover to separate enemies from the main group helps in many encounters.

The monsters will initially be more resistant to CC than you are. This is by design since their access to abilities that help them game the saving throw system is much more limited than the players'. Also, they just flat out aren't as smart as the player, so they need to be a little better defended to provide the player with a more complex puzzle to solve. The importance of having a party with a wide variety of status removal options is not to be underestimated here as well.

With specific reference to the Baron of Bones fight: is was one of the hardest fights in vanilla when addressed on level. This is due to the number of enemies involved, the very problematic arena in which the combat takes place, and the obnoxious dialogue that initiates combat and makes it hard to engage properly. These difficulties remain in EE, almost unmodified, so the way to win lies in having the "correct" tactics. You need to pull this group down the stairs and into the courtyard, preferably with one character, where your party can use LOS and the gate chokepoint to control the flow of combat as well as benefit heavily from any AOE that you may have at your disposal. It's especially good to paint the stairs with ice and electrified water to force CC on members of the group as they try to give chase. After that, you just need to kill something as quickly as you can while keeping as many monsters CCed as you can. Often, I will try to kill the jester first, otherwise I am also happy to snag a mage. The absolute best would be killing Annah first, but that is extremely difficult, if not impossible, to manage.

Alternatively, Baron of Bones is 100% an optional encounter, so do not feel bad skipping him and then coming back later. He will scale, so the fight will be challenging when you return, but you will have a much better equipped party, so likely the fight will feel a lot less unfair since you have a bigger toolkit to deal with various situations that arise during the fight.