IS2 and Tigers main strength is HP pool and armour not the damage they do.
Jackson is unique as its very squish and its also the only heavy hitter USF have.
To summarise I disagree with OP and think that's a bad idea.
I'm seeing a lot of people saying that 85s wouldn't work nondoctrially because of their stats (25% more HP is the main problem with them).
I don't see why the t34/85 couldn't replace the t34/76 if the 85 had the same HP pool as non-doc mediums and the same armor as a t34/76. If this were the case, it would enter the battlefield with a higher AT power than the P4, with less AI (no mg), less scaling, (no skirts), and less armor.
This would give soviets a nondoctrinal medium that they can use all game, while only slightly changing the matchup between ostheer t3 and soviet t3. If you just look at their stats, the main things that give t34/85s teeth is their HP pool and their access to mark vehicle (which is only available in 3 doctrines).
This seems to me the best idea to base the rework of callins on. Maybe couple this with a rework of the Stug g into a medium TD (weaker than JP4 and SU85, stronger than su76).
The t34/85 would be swapped in all doctrines with t34/76s, so that the doctrines have a stopgap, cheap tank to fulfill a "meatshield" role in the soviet army.
I don't have problems with Flack truck in general however I think its gun should be an upgrade. It's not OK to get such a powerful unit for the price of teaching.
Have you seen the vets of other factions as well then. you complain are not valid vet 5 is balanced because its nearly impossible to reach vet 5 and okw units do not start with vet 5 . volks only become decent when they reach vet 5.
Why not get rid of the Vet 5 system all together? Vet 5 is just a broken mechanic just like vCOH Wehr was able to purchase Vet 3 on all units.
Have you seen what they can do as well? At optimal range, in green cover, they melt infantry like hot knife butter, yet I can read they're worthless from majority of OKW players. At Vet5 they are just insane.