The 76mm HE wasn't any more powerful than other tank guns and army designations do not prove anything (Panther 'medium' tank or light/heavy cruisers are just some examples). Stug literally stands for Sturmgeschütz (Assault gun) but that one can't fire HE shells? Su-76 wasn't extensively used for indirect fire either because it was rather ineffective at it.
So in conclusion the AI barrage is just a game design choice to add variety. Which is good and all, but it shouldn't be that good because it would make the vehicle too effective for its cost.
As for its AT I agree I would've preferred to see a penetration nerf instead of ROF but apparently testing showed that it wasn't a very effective nerf.
1. the 76mm actually had superior HE filler and fragmentation to the PAK 40 on the stug as it had less burst filler in it thanks to the much higher velocity...meanwhile its HE filler was only slightly smaller than the 75mm m3 of the sherman but had superior fragmentation capability... more or less equal to the 75mm m3...
2. it could be used as indirect fire but its shells werent comparable to the larger guns which gave it inferior performance compared to howitzers... still id say those shells are better off than mortar shells and the game does reflect this in many ways... regardless it was used alot in this role when heavier artillery such as 152mms and 122mms were unavailable
3. nerfing the SU-76`s AT along with nerfing its AI simply renders the unit pointless when T4 arrives... even the T-70 and the m5 halftrack at least have niche uses they remain good at...but the SU-76 is fully obsolete by T4... at least buffing its anti infantry capability or giving it a role of anti medium should give it an overall purpose even with T4 around...