If the su76 is trying to not get shot up by your 222 its not barraging your pak. Additionally the main issue with the su76is when its blobbed, and unless the game was patched in the last few hours blobbed armour doesnt move effectively in the least. I dont expect you to solo a pack of su76 with a single 222, but you cam use it to disrupt them. Relative to cost the 222 is your t34. Its enough of a threat they need to account for it but cheap enough to lose if it means securing a kill. You can block with it, its gun is effective against them, especially their rear.
And i never said the su76 doesnt need nerfed (i agree with vipper that a pen reduction and tying barrage behind vet would be a good direction, maybe even tying the zis barrage behind vet too) simply arguing that its ridiculous to only look at the gun stats when comparing the stug and su76 and ignoring that the su76 is THE glass cannon incapable of bouncing anything heavier than a scout car and being unable to take many hits either.
But you are never going to get close to the su-76 (blobbed or not) because every sov player has guards these days, if not at nades, mines, or some other form of detterent.
In theory, the 222 is a nice distraction, in practice, it's going to die before even getting close to the su-76 (and not neccessary by the hand of said su-76).
There is also the issue of range (50 for stug, 60 for su-76) which means su-76 has alot less to fear.
I'dd rather have it's range reduced to 55 or 50, also making su-86 more unique.