I'd be all for removing smoke canister from scott or 400HP -> 320HP (2 shot kill), but I'm not really a fan of giving the stuart smoke. Its problem IMO is its main gun doesn't do nearly enough damage to vehicles. It survives well enough already being practically immune to luchs and 222 fire. Only the puma and larger can harm it. So I don't really think it needs to extra suvivability. I'm more of buff the AT damage, lower the armor and AI damage guy.
It doesn't need extra survivability against light vehicles. It's fine in that ecosystem.
Its problem is, unlike the AEC and Puma, it becomes obsolete once the medium tank phase hits. That extra armour means little against a medium tank's gun and, as you said, its main gun is almost worthless against mediums and up.
The tooltips and loading screens suggest the Stuart's a control unit: its damage sucks but it can swing fights hard when supporting a medium with Shell Shock and Point Blank Engine Shot.
Those abilities are questionably useful when the Stuart rarely survives the attempt. A smoke canister would make those abilities more practical to use.
Your suggestion of reworking it into something more like an AEC would also work. The AEC and the Puma are proven good designs for AT lights.