P2 not light tank? I know only one allied nondoc hmg with ap rounds - USF M2. I also know 2 non-doc axis HMG with AP rounds. One from them in T0.
One does not need a Su-76 to counter a PzII even the T-70 counter it.
I don't know how less potent su-85 make su-76 more potent. I bet, you will see zis-walls or new shine of M-42, but not su-76. Strong su-85 is counterpart against P5. How you suppose to counter P5 with weak su-85? Or which unit must be hardcounter to it in your opinion? Also don't forget OKW P4 with stock elite armor and stock kingtiger.
We are drifting of topic now but the SU-85 has on of the best AT guns in the game and it is very effective vs all axis vehicles.
One could easily design it to counter Heavy tank effectively and medium tank less effectively.
For instance one can lower the accuracy.