As Soviets, if you're playing T1 you can rely on PTRS to fight LVs. If you're playing T2 you have access to the ZiS which has better late game scaling. But with Axis' weapon upgrades kicking it at around 4-5 minutes, your infantry will start losing the fight and the Su76 will not help you in that regard. So you will always build the T70 to carry you over that 10 minute drought until your infantry becomes competitive again. Additionally, the T70 can take on all Axis LVs if necessary except for the Puma, which in turn only is built in response to your T70. And nobody will to T70-(Puma)-SU76, especially since the SU76 is a shitty choice against OKW. Against OST LVs, your T70 will do fine. There is just no niche for the SU76 in the Soviet army.
So the unit is fine, but it is inferior to almost every other alternative that the soviet faction has, on TOP of of shooting yourself in the foot for not getting a T-70 or tier 4
If a vehicle doesn't fit any role, or does them worse than any other option, then the vehicle clearly needs some work to make it better, and the SU-76 fits just that criteria. Yes, its worse than the zis, its delays your t-4 for no gain, the vet1 ability is simply stupid, and so on. All it needs is a push, and it could be a vehicle that is useful throughout the entire game, and not for a time when it isn't even available.
Ironically that there is a fear of the SU-76 being spammed, when the only way you can utilize the AT of the su-76 after 12 minutes is to make 5 to 6 of them and pepper the enemy with low damage shots (and yet 3 stugs do the same task but way better than the su-76 ever will xDdDdD)