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I have been disagreeing with your assessment of the Su76 performance, and the reasons for it being used infrequently. I will address that again, then my thoughts on your suggestions.
Yes an ostheer player would absolutely use a unit like the su76 if it replaced the stug. The issues of it getting flanked and punished are ones that can be overcome through proper support. If a StuG gets flanked by an AEC, it will die just like if an Su76 got flanked by a puma. Use the Su76 as conservatively as you would an Su85 and you cant go wrong. At least the su76 has the speed to pull back from a panzer 4, the 85 doesnt.
You use the Su76 for zoning and area denial. You dont try to kill tanks with it, you put them at low enough health where they cant charge without getting a guaranteed snare. You dont need to massacre infantry with the barrage, you just need to force an mg to retreat.
As for your suggestions:
I am against the first one as its too exploitable (eg enter the mode when the barrage is off cooldown, fire the barrage, then exit the mode) and a costless barrage on a cheap vehicle with AT as decent as the Su76 is not a good idea.
Your second suggestion... I'm not entirely sure how that's different than the current barrage unless you mean a direct fire barrage and I still dont know why it should be changed like that.
Your third suggestion has merit. Soviets have a severe lack of non doctrinal smoke options and a vet 1 smoke barrage could give it a lot of utility. I do however worry about the amount of smoke a pair of su76 could lay down at long range so if this is implemented, the smoke barrage would need a long cooldown.
AP rounds- While it's not horrible, I do not think the ability would be worth the micro tax or cost.