I have seen several instances of the su-76 in both my games and in that of other people by now. I got to say that it is a pretty underwhelming vehicle, confirming my previous beliefs on it.
It lacks dps and durability, even the range aside, the stug completely destroys this machine (and can straight up rush into it). In conclusion, you delay t-4 and a t-70, all for a low damage cannon. Not worth getting.
I use the SU-76 regularly, and I think it complements perfectly with t1 penal builds. In games where you are kinda low on fuel, you can slowly but surely get multiple SU-76, and I think they are devastating in groups of 3-4 because then you have a "tank blob" that will effectively trade in most fights, because if the enemy attacks you and loses a tank, usually you will not lose more than one SU76.
If fuel income is good, however, I would just tech straight for T34 and SU85 of course. Or get one only so enemy p4 rush doesnt push you off the map