Subscribe to download
Never Generate Relations

To do it badly, very easy. Instead of setting BaseGenerationChanceFactor to always return 0, you would recreate the function and make the HostileTo check multiply num by 0 instead of 0.7. You're more likely to run into compatibility issues than with this mod, but not by much.
You'll still have hostile pawns arriving with relations though, since they can just get the relation while they're not hostile, switch faction, and then spawn on a raid.

Pawn relationships add depth and realism. It's a nice touch, at least in theory. Though, there are some pretty big downsides to RimWorld simulating them, such as Colonists suffering the death of a relative or the huge lag / performance hit on checking each & every raider to see of one of them is related to someone else.

Rather than disable randomly generated relationships for everyone (by setting it to 0), I would find it far preferable to just prevent any and all relationships on pawns hostile to the Player Faction, while still allowing it for pawns who spawn as not hostile (neutral or friendly). By using such a rule, raids - whether spawning as an individual or in huge waves - could still skip the relationship check.

Theoretically, how difficult/feasible would it be to code it like that?

switched to this mod and with this mod there are perma glitched [ERR] quests no more

This is better option than "No Random Relations" because it seems like that mod works retroactively (removes relations after pawns spawn) so it works but doesn't really help with initial lag when pawns spawn (like raiders) but this mod seems like it works proactively so it helps with the initial spawn lag. I use both to ensure there will be no random relations left.

I haven't played for more than a year but when I did the best bet was to use the endling mod https://steamcommunity.com/sharedfiles/filedetails/?id=3011354315 and add the trait on to your original colonists. There were some hugslib mods that purported to do what you're describing, but in my experience none of them worked.

Is there a MOD that prohibits original colonists from having random relationships?

This is a godsend, would love to send you beer money. My colony has been on the brink a few times now; every second raid consists almost entirely of long-lost relatives because my pawns have family trees that would make Genghis Khan jealous.

Ahh gotcha, thanks for the for all the details about how this all works, especially for the "premade" pawns and raiding visiting other factions, definitely keeping it then. Cheers mate!

1/2
If they have listed relationships that you can see ingame already, then yes.

When you generate the world there are like 5-15 pawns of each faction, plus some will get made and saved from caravans and fleeing from raids and the like. If you turn this mod on on an existing unmodded save those pawns can still have relationships if you find them again later, but recycled pawns are the distinct minority and raiding a enemy settlement is basically the same as being raided in terms of pawn generation. So it's possible, but unlikely.

2/2
Also, the in-group bias for spawning with relatives in your own faction isn't usually high enough to overcome the fact that there are just a lot more pawns not-in-your-faction, so chances are high that that pawn's relatives will not be in the same faction*. Pirates/Empire it's a little more common because of the tech level disparity, but for other pawns chances are high that their relatives are just in wherever.

*This is basically the reason the main thing people install this mod to disable happens so often in vanilla, even though there's an additional check to make spawning with relations to the player's pawns less common. You can easily see it in action by going to an unmodded embark screen and rolling your colonists and seeing how many of them have a relationship with someone who isn't in your starting 8 (and is therefore in a different faction).