That does not change the fact that their should either be a distinction in design between heavy and medium TD and Panther/SU-86/FF/M36 should all be in the first category or none of them. This is a simply design choice unrelated to balance.
If that was true the Stug would not be used either for the same reason.
JP4 is not used because it is very expensive, takes too much pop and can easily be circle strafed and die if only one is build.
And once can easily say the same about the SU-76/M10, "isnt used cuz the Su-85/M36 overlaps and is a safer choice"
You cant ignore faction traits such as having stock heavy and super heavy armour. When attempting to balance td's and units such as the panther. The axis do not face stock super heavies. They already have good at power to deal with mediums. Panthers should not also excel to much or more vs meds then it already does.
The su76 is fine at best vs ost. It isent fine vs okw. 234 armour out of the gate on meds will see to that. Puma will see to that. It will circle strafe or kite it to death. The su76 is more a light td then it is a medium td. Hence the su85 who is stronger more durable and its gun can move is a safer choice.
Stug and jagp4 both do a lot better at being a medium td then the su76.
Ost panther usualy arrives later there is a bigger time frame for the stug to be effective in. For okw jp4 (wich is about the price as the su85) and panther are in the same tier and since the panther went from about 9 sec relaod to 6 second reload there is no reason to build jp4 since it buffed the panther vs mediums.