Buff the 222 too much and it will become OP, I think it's in a decent spot atm. Just slap a fuel price on the quad and make the T-70/SU-76 a tad bit more expensive.
The problem with the 222 since the start has been its lack of armor. The minor health increase didn't remedy that, nor did the munitions saved from the free upgun.
Small arms fire just tears through 222s. They don't need to be immune to small arms, they just need enough armor to reliably prevent damage at max range from rifle and SMG fire. Essentially it just needs to be able to engage enemy units without requiring extensive repairs afterwards. It can't exactly cart around its own repair crew like the M3, M5, or any USF vehicle.
Paying for the upgun again would be just fine in that situation. Hell, maybe even the health reverted too just so long they could mitigate damage from weapons weaker than LMGs.