I do apologize, as I could not be bothered to read everything on this thread. My thoughts on the SU-85 is simply that you cannot do much to it at the moment because Soviet Tier 3 is so terrible. They need to change the other T3 units for both sides really before a lot can be done in my opinion.
At the moment it feels like there really isn't much need to get niche units such as the Ostwind (good damage to light vehicles, aircraft, and infantry) or the StuG (which isn't very good at it's niche currently anyways) when you can get a Panzer IV that does everything very well. Let alone two! Very cost effective with even only amateur micro, and basically the last step to defeating an SU-85 is out flanking it. And I'll go all the away around the damn map if I have to in order to do that.
As for Russian T3, the only other unit I really see much of is the SU-76 but it too is kind of like the StuG of their faction. Not good at filling its role. I do not play the Soviets enough to be confident about suggesting things, but perhaps the SU-76's barrage could be more potent? Longer range? Make it more of an assault gun? I don't know, something.
Of course, to fix the other units, the Panzer IV and SU-85s will need changes as well. It goes hand in hand. Fix the other units to fix the PIV and SU-85, fix the SU-85 and PIV to fix the other T3 units.
P.S. I would go as far as to argue there isn't too much "OP" about the SU-85, and only some "OP" aspects about the Panzer IV. My problem isn't necessarily with over/under-powered but rather how stale the game becomes due to these issues. Like in vCoH, we want a variety of strategies, many different possibilities and outcomes.
If you do not believe me, I recall playing with a new good friend of mine and we literally used the same strategy over and over again in 2v2 as the Ostheer. He'd get the Opels, I'd get the Elefant doctrine. I eventually got bored and told him that it wasn't his fault, but I just couldn't keep doing it as it was getting rather boring.