Do this in order:
1- Let's wait to see where we actually are after the tournament ends.
2- Delay a bit more the heavy call in tanks.
- Recheck all TD xp values and adjust accordingly.
- If vet pen based is to be changed, it could be tweak to be equal to armor buffs received by Axis.
3- You can also swap most of the offensive vet buffs to vet 3 and replace double offensive buffs on the same vet level for more defensive/mobility ones.
4- If after all this, the Jackson still requires a change:
HVAP becomes a vet0 ability with no munition cost and is a long time to switch toggleable munition.
RoF on HVAP nerf to be equal to that of FF, lower accuracy values (which should in theory be enough for heavies)
Default gun range reduced to 50 while moving. 60 range when still.
The problem with Jackson, is mostly a teamgame issue. I don't think you can reduce the HP and return to status quo of before, cause the clutches which made them work no longer exist in the game (for good). While it would be ez to make it a FF clone, i don't think it would be approved.
Since the unit already has HVAP, i think this could be the basis to make it work and give it a clear weakness compared to FF and Su85.
As i mentioned before, you could try giving it 2 type of munitions at vet 0 and make it work as the default Sherman.
Then you could add mobility based buff in vet, and make the reload/accuracy/pen based vet buff only affect a certain type of the munition shells.
For ex: vet 2 which is acc + reload could go towards the default gun while vet 3 which is pen + reload towards HVAP.