1) M36 is being OP for 3 years now...
2) If one want to stick with a "sniper" design why has to simply apply similar logic with "sniper":
Reduce mobility since a sniper is not faster than infatry
Use an inverted DPS profile similar to sniper DPS profile reducing ROF (or even accuracy and penetration) so that unit is an disadvantage when in close range
3) But there is little reason to use a "sniper" design since the part of the problem is that in the current implementation the unit has the mobility to stay at max range and little reason to move closer since its DPS is very high at max range and improves very little at closer range.
The Jackson performs like I'd expect a dedicated tank destroyer to at long range. I've got no complaints there.
The problem is the Jackson
also has solid performance close up, which constitutes a big increase to its 1v1 power. Fireflies and SU-85s need to play much more defensively because even a Panzer IV can kill them if it gets a good angle on them.
The Jackson's fast rotation and solid rate of fire let it brawl with mediums on even terms, allowing it to play much more aggressively. You can dive into a base to secure a kill with a Jackson. That'd be madness with an SU-85 or Firefly.