The thing is that the tank could be made very good against infantry without making the gun into an unfairly good one shot boomstick. Just do one simple thing;
Introduce every machine gun the Stuart had as fully functional.
One .50cal on the roof, and five (yes, five) 7.62mm brownings? That should mulch infantry all day.It has more machine guns than crew members, that's how much it loves MG's baby.
While the idea of that gave me a most normal boner, I'm regrettably reading that it only had 3 .30 cals make it to production and it didn't use a .50 (initial M3 Stuarts did have 5 M1919A4s, but it got reduced to three definitely by the time the M5A1 got made. Any variants that I could find which had a .50 cal had it in place of its gun turret)...probably because it couldn't fire them all off at once anyway, like you said.
I do agree with making it better against infantry with its MGs though. The less main guns squad-wiping the better, but that's no reason to keep the Stuart bad.