The StuG is in medium tank tier with the Panzer IV. The IS-2 is in medium tank tier with the T-34/76, the Pershing is in medium tank tier with the Sherman, the OKW Tiger I is in medium tank tier with the OKW Panzer IV.
Heavies are tied to medium tech. Until they get a ~100 FU teching barrier separating them from mediums, they're medium-tier generalists. Trying to balance them as some sort of tier up is like sticking the OKW Panzer IV in Mechanized.
The StuG is in an equivalent tech tier to the Jackson. It's cheaper, but it's also a 50 range casemate with less health and penetration.
It shouldn't do as well against heavily armoured units, but it should not be almost completely neutered by a generalist in the same tier.
The SU-76 is a little more difficult. It's not for killing light vehicles: the SU-76 doesn't counter any light vehicle the T-70 doesn't already. It used to be a hybrid light TD/artillery tractor for more defensive play. Then it got a munitions cost on its barrage to stop teamgame SU-76 spam, and since then it's been a dead unit outside of pre-rework heavy stalling.
I think what the SU-76 really needs is for the balance team to decide what it is.
The problem is the Jackson is it doesn't have the close-range weakness the other 60-range TDs have. Turning it into some sort of anti-heavy silver bullet is an idiotic design choice. Doctrinal, medium tier heavies should not demand a 140ish FU counter unit that isn't good against anything else. That's a perfect recipe for people rushing heavies every game.
Heavies are currently medium-tier vehicles and until we stop treating them like they're a tier up they're going to play like a medium tank in the light armour phase.
Heavy timing is still under construction and I do believe a tech to unlock them is necessary. Something woth a cost but more importantly a long "build" time and it should be in the tank tier as it would block out armour production while it's being researched so they need an army to stall for them instead of simply being rushed.
And regardless of the exact tier, they lie in each factions top tier (or close enough for Ost) and heavies basicly ARE like a medium tank in the light phase. That's the point, but that's why they are limited to 1 now so that they may be zoned. Additionally both the StuG and the su76 are not in their respective end tiers. There is still more to thier factions and neither of those tiers are SUPPOSED to last to the 1 hour mark. Both have room to escalate and heavy armour ensures that they will.
I agree that the medium TDs shouldn't be neutered against heavy tanks but we do not want the return of spamming cheap TDs to ensure the enemy can not use any armour regardless of size. We have been there and it was cancer. Thus why I suggested adding deflection damage on full tech. The issue with these units now (less so for the su76 due to the barrage) is that they don't scale past being AT units when they become more obsolete. Light vehicles can fall back on their scouting should they remain alive when they lose their prime operational time, these 2 units less so. Deflection damage would allow them to contribute against heavy armour without overstepping into heavy TD levels.
For the su76 I'd try reducing the cost of its barrage with vet instead of tracking so that it can continue supporting with less weight on its AT.
If the Jackson isn't the answer to everything on tracks or wheels anymore it wouldn't need such a high price tag but it could be given scouting capability if it doesn't do fucking everything as well. It used to have it and it could again I feel.
Again, heavy timing is still under the scope so making them their own tier is more than possible but saying they are medium tier without accounting for the fact that they are nearly 2x the cost of a medium and also hard limited and doctrinal is disappointingly short sighted. In the time when players are already teched for armour saving for a heavy puts you at a 2/1 tank disadvantage much like going 2 cons vs 1 penal reduces your field presence. At that point holding out can cost you as much as a tech cost can by bleed and lost ground. It's not as black and white by that stage.