The flame halftrack upgrade is tricky, it used to arrive very early and all you would see is flame HT every game. However, it's never seen anymore after the nerf. My suggestion would be to remove the BP2 requirement, but remove the ability to reinforce once upgraded (same for Soviet HT). This would still make the FHT a nasty vehicle to deal with, but not a unit that has to be focused down ASAP or else you will lose the game.
It would be nice if the 222 didn't explode in 1 hit to 30 munition mines too. It just needs a 10-20 health boost for that, since a lot of light vehicles now (post-WFA) seem to have high health.
Mine vs vehicle balance is fine I feel. Other scout cars have the same health as the 222. The Kubelwagen, M3, 222 and american scout car (forgot the name) all die to one 30 ammo mine, be it a soviet or OKW mine. Other, non-arty light vehicles all have enough HP to survive a 30 ammo mine, but get insta killed by 60 ammo teller or the american 60 ammo mine. These mines however can also 1-shot much more expensive units like the T70, SU76, M8 etc.