The best idea I've got is making it 480 HP, but the problem there is that makes the KT, Elefant and Jagdtiger two-shot it.
Missed this before, but lowering its HP has been effectively vetoed by the balance team. As you pointed out, it makes it far too fragile vs. super-heavies, which are very common in team games (why it was increase to 640hp in the first place).
A unit that has to stay at long range can't dive and brawl.
The Panther's advantage over the StuG is its ability to dive into enemy lines to secure kills. The casemate StuG can't do that because it's a casemate and can't let anything get behind it.
That's what makes the Jackson so powerful: it can dive and secure kills where the SU-85 and Firefly can't.
I don't think its ability to 'dive' is entirely what makes it OP; it's the ability to push units off points in combination with the strength of other USF units, and in combination with it being an excellent defensive tank.
If USF is attacking, they can keep the M36 at max range and consistently chip away at enemy tanks (except super-heavy TDs), forcing them to retreat. If those Axis tanks back off, that means USF rifles (or other infantry) can come in and wipe everything else, since OST (and to some degree OKW, but much less so) is so reliant on vehicles to counter infantry.
Meanwhile, any counter-attack can be easily stopped, as the axis tanks won't be able to dive (can't catch up) the M36s, and as such, won't be able to provide infantry support.
Take that away and it's a fast SU-85 without self-spotting.
And 50% more moving accuracy (0.75 vs. 0.5)
And a turret
And HVAP
And incredibly good vet (325/390 vet 3 pen + HVAP!?)
And a crew
And the ability to bypass pop-cap
What's the downside?