"Only" lets it have the damage of the ZiS (Or The P4, Cromwell, Sherman, SU-85, Jackson, etc). 160 damage is an important breakpoint as you well know. Not really sure why you're trying to downplay it.
The Stug gets better mobility because it has lower range. That's the tradeoff.
The SU-76 gets scaling through an improved barrage range and cooldown, alongside better accuracy/reload. (and ultimately better penetration values than the STUG).
reaching veterancy 3 with most units is a long process, you are only getting the 160 damage at veterancy 3, which requires a lot of care from the SOV player to get to there in the first place. stop downplaying the fact that its a VET 3 buff, on a fragile unit that has weak stats to start off with.
the tradeoff makes no sense seeing as the su-76 is ENTIRELY mobility reliant to survive
the "scaling" on the su-76 is entirely redundant because you're getting better barrage AND better AT with the zis-3, and you also skip out on the T-70 that can go against LVs, getting it later on is entirely pointless because of the su-85, but thats okay.
the stug scales way better for the virtue of it's good AT stats and mobility in a rather cheap package, the lack of AI is irrelevant because you got two AI tanks that do an excellent job and the lack of range is because it could probably 1v1 su-85s with 60 range, the reload speed is definitely not slower than that of the su-76 and the accuracy is good enough. it is a really underappreciated tank by the community, but I digress.
i don't know why you're adamant about the damage, it isn't a vet 0 dmg buff, but a vet 3 one, which promotes unit preservation on a fragile unit