The hole in the theory that they are not meant to fight each other, is that SU-85 is meant to counter the Panther and Ostheer have no stock vehicle to counter the SU-85 so when it comes to vehicles the Panther is their best bet.
Ost has an excellent Pak, even the Stug faires well on paper, and then there are Panzergrenadiers that can deal with TDs very well. The problem is that the last two units have their own problems and therefore are not build which reduces OSTs current options to Pak and Panther. The problem is not the Panther but Panzergrenadiere and Stug in my eyes. If we also look at 1v1 frontal assault scenarios only (like you implicated with the SU85), the Panther does well against Jackson and Firefly (cost-corrected).
To clear things up: What is your suggestion? How much do you want to buff Panther / nerf SU85?
I think it is better so spread out the AT capacity over multiple units. We can see with USF what happens if the AT gun is trash against actual tanks and the bazookas are only mediocre: USF must completely rely on the Jackson from the mid-game onwards.