Gents, it seems I have solved the Stug problem and the range 2 problem in this case.
I did tests, and the result was surprisingly adequate even for a maximalist pessimist like me
Problem 1: StuG and generally small inf guns at range 3 is not realistic. These guns had dual role of a field guns and a mortars at the same time.
Problem 2: In artillery configuration StuG is too vulnerable, which is also not acceptable since it is the same vehicle needing no time to change target. While mortar infantry would certainly need more time to set up mortars it's reverting at turn end.
Problem 3: StuG Being class Tank is incorrect. It's a support weapon, most of the time fired from a position or marched in support with infantry. Tactical role and doctrine is not of a tank's. Tank class distrupts upgrade chain of StuGs in particular.
Solution:
a, StuG (and similar vehicles) must be
AT-class to provide support fire in defense of the infantry it supports. It does not matter if that particular StuG had AT-gun or howitzer, in a support role it would fire at armor anyway with HE or HEAT even.
b, support of attacking infantry is done by ranged fire, previously by switching to ART type. I have changed this to
instantRevert to retain to AT type by the enemy turn so it can provide support AND defend itself in it's original config.
Issues not resolved yet:
Infantry guns (or battalion guns) are - at this scale - integrated part of infantry units (like mortars).
As a matter of fact light AT guns also fall into this category! Separately they are both vulnerable if not covered. Omitting them is not an option, I think we can agree on that. Giving them extra range is not realistic and is not a solution anyway. I'm doing tests with units having weapons/switches as special attack or integrated guns... It's also an OoB question of the faction.