The quickest way to make this unit passably useful is to increase the health to 400.
400 hp is the same as the StuG (another assault gun). It's a good break-point because it means the SU-76 takes three shots before it dies to AT guns and Panzer 4s rather than two shots.
What is the point of an AT assault gun that cannot trade even one blow with an enemy tank without needing to immediately panic because it will die on the next shot? This is a huge problem for the SU-76 as a soft anti-tank counter. It is more likely to get picked off by what it is supposed to fight.
I think you could buff the health of the SU-76 and it could see play as the first unit you get from Tier 4 if you rush the tech. It would not be a unit that you would build beyond a timing push (much like the T-70 which is rarely beyond the first one) but it could be okay at that.
Buff the health and then look at the unit. Does it need penetration? Does it need more damage? Does it need more utility? Does it need anything?
I suspect it would probably need a smaller buff in addition to health but not a whole lot.
StoneTheCrow1:
Assuming equal fuel you can basically get 2 SU-76's for the Panzer 4 but the idea is that you use your earlier fuel tier units to push the field a little bit as you will have the first SU-76 well before any tank you can use him to barrage infantry or support weapons and kill any half-track, scout car etc. Hopefully allowing you enough leway to squeeze out another SU-76 to help fight the panzer 4. You don't need the third though if you get an AT gun if you went tier 2.
I wan to talk about this idea.
This is what a T-70 does when it hits the field early. It crushes enemy light vehicles and gives you significant map control. It drives away problematic infantry and has the potential to wipe them out.
A rushed SU-76 can never really do this. The barrage is clumsy and it telegraphs the attack with an icon over the unit. You can choose to fire a barrage from outside of the enemy's line of sight but it will almost always miss the first shots at such a range. You can choose to bull-rush enemy infantry and surprise them with the barrage at close range: which puts you in danger and is still telegraphed by your rush and then the icon over the tank. Both options are bad and have a 60 second cooldown.
The SU-76 cannot seize map control. You lose map control and potential early game power by advancing your tech and buying this unit. That costs 240 manpower for the building + 270 for the SU-76. That's over two units of infantry you could have had out earlier and fighting.
It is unrealistic to expect the SU-76 to do anything in the short term for map control. It is a long-term unit that extracts value slowly through barrage and using its 60 range cannon to snipe away at enemy armor.
The SU-76 has no late game though because it is too easy to kill while doing its job. This is the paradox of the SU-76 we have today. A long-term unit that needs to be rushed and has no real future.