For reference, I'm sitting around at the early midgame on the current build, with mag weapons and plate armor, both Squad Size upgrades, but no Colonels yet.

I'm really sorry, DerBK, but in my opinion, Cannons are not good enough in any given role to outclass the other primaries. Shotguns are extremely powerful in CQC at the expense of ranged utility. Rifles are versatile at the expense of damage output. Snipers are powerful AND accurate, at the expense of mobility and close-quarters utility. Cannons, as currently designed, get three strengths: ammunition count, environmental damage, and (potentially) two shots per turn. As tradeoffs: EXTREME mobility impairment (even Snipers get to move + reload, but Cannons do not), and significant accuracy penalties at all ranges if you use Spray, which is presented as their ostensible "niche".

I'm trying to utilize Cannons. I really am. I'm deliberately forcing myself to use Spray as much as possible, often trading what I consider a more optimal play for double-sprays, just to see if anything of note or usefulness happens. I STILL have yet to see any real, useful environmental/cover destruction to speak of. My Scout, wielding a ballistic rifle at the time, has accidentally destroyed an enemy's cover with a standard shot exactly once - more times than any of my Cannoneers have ever managed, with deliberate multisprays, in every circumstance I could contrive to attempt it. The Cannon's environmental damage is simply not reliable enough to even be noticeable, let alone a tangible "strength".

Now, what you do with your mod is up to you, of course. But, I beg you, consider buffing Cannons a bit, or at least reexamine their purpose. I think you're on the right track with the "Volume of Fire" angle for their niche, but the tradeoffs are so crippling as to render them essentially pointless when set up against the other primary weapons.

In my opinion, what would make Cannons an interesting AND viable option as an alternative to the other primaries is:

a) Keep the significant accuracy penalties if the Cannoneer moves and shoots.
b) If the Cannoneer stays put, ALWAYS let them fire two shots, and let the first shot be a regular old standard shot with no accuracy penalty (beyond the usual).
c) AFTER the standard shot, allow the Cannoneer to either Spray with an aim penalty, or Suppress a target.
d) If this seems too powerful, perhaps consider a requirement to spend a turn "setting up" the Cannon somehow.

I know you've expressed some hesitancy about cluttering anything with too many innate perks, but I think it's a system worth considering. If you wanted you could drop the Suppression option, of course. But the Cannon needs SOMETHING, as it currently stands, and I believe that tactical crowd control could be exactly the niche. Suppression, contrary to what some have said, is a legitimately useful tool (though I've always thought, as a kind of "single-target" Overwatch, it should not be subject to the -15 aim penalty it has in Vanilla), and has saved my soldiers from injury or death many times. The problem is that Suppression is very rarely worth giving up your standard shot.

So, give us options! Let us have our cake and eat it, too. Let us keep the Cannon's already mediocre standard shot and then rain down additional fire.

For your consideration. Thanks for being an awesome modder, DerBK. I'm confident you'll find a place and a role for the Cannon eventually.