Loving the mod, a natural fit for the game and have been using it to shore up my defenses.

Only input I have is that the 2 MKs of cannon are kind of superfluous and don't mesh with the vanilla game design of "one weapon, many upgrades" through ammunition, tech and now, quality.

I'd suggest you fold both cannons into one as getting MK2s is not hard anyways. Then introduce a series of techs to boost howitzer range (if you want to be cheeky, quality could boost it too for sillypilled pseudo-artillery) with the base gun having about the range of the MK1 and the techs boosting that range to the MK2 or perhaps beyond with utility science. Tying it into vulcanus tech to push further and introduce tungsten into the ammunition would be cooler still.

The only thing you lose with this implementation is the different minimum ranges which don't pose a terribly interesting design consideration as you just put them further behind the walls anyways if you want them to be able to shell closer-in targets.