The su76 doesn't need to have an always build role. As it stands it's a good emergency TD. It's got good pen and ROF and it's barrage means it isn't wasted when you get your proper AT out. It's semi niche but doesn't require a whole rework..
Make it deal 160 with vet and call it a day
No it is not even reliable if you use it as an emergency td.
120 damage means it need to deal 6 penertrated shot to kill a panzer 4, consider that its penertration has been lowered to 180/170/160 the shot may deflect if luck isn't with you.
Its rof was toned down long ago, now its reload time is 3.9-4.4 so it needs at least 24 seconds to fire enough shots to deal with single panzer 4 alone.
You may argue that su-76 can't be the only AT unit on field, then assume that there is already a zis. In a 10 second window both zis and su-76 can fire 2 shots, that is 560 damage in total, able to get a stug but not a ost panzer 4. So how can you call it "good emergency unit"? In the same senario double zis will definely perform better.
And fausted su-76 can be killed by 2 tank shots, 3 shots if not fausted. Does zis-3 die of 3 panzer4 shots?
If su-76 means to be good emergency unit, it will need a health buff or damage/rof buff. Health to 560 / vanilla damage to 160, since it lacks turret. Remove the barrage and make it a soviet stug then, but that is absolutely not the design philosophy of it.