I never said it was a simulation. The game however clearly tries to be a abstract representation of a ww2 battlefield so some degree of historical argument is valid. Otherwise why not give every German a STG and give the Americans lasers since historicity doesn't matter?
You are correct that t34-76 where upgunned to 85mm to better deal with Panthers and Tigers. "dealing with in'' this case is more trading shots then active hunting as the commander name would imply. The ISU152 that you proposed would fit better in this regard (still debatable though). As far as gameplay is concerned I you must have a medium tank in this doctrine then the m4c with it's HVAP rounds that can more reliably pen panthers and buff nearby tanks rate of fire would fit and synergize much better then the already over saturated t34-85. The overhanging discussion here is how do we define a commanders theme and how strictly should it be followed.
The commander’s theme is “tank ambush”. This is achieved by deception and heavy damaging abilities that rely on such deception.
More specifically, you have PTRS conscripts which have camouflage and can saturate areas with ultra cheap 10muni stun mines. Tanks and tank destroyers also get camouflage to position themselves to strike at enemy vehicles, either by ramming (76) or shooting (su85). The damaging abilities are the conscript grenade assault, T34s coming out of camouflage to flank and the carpet bombing ability.
Here we come to the ML20 or your B4 proposal. You cannot know exactly where an enemy tank would hit a mine allowing you to pounce, hence you’d have to reposition the B4 to actually get it to fire, which is a 15seconds minimum process from starting to use the B4 to actually landing a hit. Most engagements are over way before that. Thus the ML20/B4 does not fit the core of the commander’s concept since it takes too long to deliver a hit. Now, if the B4 had a wider firing arc you could use it defensively with the direct hit ability, but as it stands you”d have to reposition it to get it to fire, which would take too much time and your opponent knowing you have a B4 would avoid close proximity, making it harder for you to utilize the ambush part of the commander since he’ll be very cautious about getting close to the B4.
Now contrast this to the T34/85, a tank that can beat a Panzer IV J 1v1 and can brawl with a Panther with favourable RNG. Such a tank would benefit immensely from being able to ambush enemy armour that’s been stunned or grenade assaulted. It would provide a huge late game lift to this commander by replacing the T34/76 as the main power unit in your late game composition, while fitting both commander themes perfectly, being able to ram, camouflage and flank.