Interesting idea. Agreed that for the SU-76 to fulfill an AT role it would need some serious buffs. Right now it does 80 dmg, 93 pen (why 93? such a weird number), and 2.2s reload. Compare that to ZiS and SU-85 that does 160 damage, 170 penetration, and 4s reload.. which is much better AT.
SU-76 is considered useless by many because it doesn't really fill a specific role, unlike many other units in the Soviet arsenal. It can take out light vehicles reasonably fast but by the time it comes out, you're facing P4s so most people opt for a safer investment that is the SU-85.
I still think light armored units like the SU76, T70, and the M5 comes a little too late thanks to the 145 fuel requirement Soviet needs to research them. Taking inspiration from Panzer Elite from vCoH, it might be better if the investment to get the initial T3 or T4 building is reduced, and players are allowed to get these lighter units right away. Then if you want to get the harder hitting units like the T34/76 or SU-85, require them to upgrade further for some MP/FU - perhaps adding up to 145 fuel. That way you won't need such huge fuel investment just to get a halftrack and it opens up more flexibility in Soviet midgame play.
This!