You can never balance P4/Puma with Jacksons because of the USF army design. USF only have Jackson that can fight Panthers while Axis has variety of options from Puma, P4 (can fight infantry!), stug, Panther, & JP.
If you want to nerf Jackson's armor or reload than give USF non-doctrinal choices for alternatives. USF late game AT options are limited to Jackson spam almost every game.
It is pretty unfair that you compare a single faction with 2 axis factions combined. OST does not have pumas and OKW P4 has not the same impact that OST one. Not even going to continue the idea since its already settled.
To nerf Jacksons armor means such a little change it shouldnt even bother a normal skill player, since as many other ones here said, if you F'ed up and a P4 got close quarters to you jackson its more than logic that your TD get a fair pounding. There is still room for the M36 escape to escape, but again it demands skill.
A good analogy is treating m36 like a sniper, is someone got a whole squad of shock troopers right beside a sniper, would it be fair that the single press of 'R' will save the sniper? I think that no one will say yes to that. Jacksons are not as fragile luckily but that doesnt mean it must be unbeatable either.