I think reworking the Stuart is the way to go. It's a really odd unit that utterly dumpsters T2 Ostheer but is pretty much useless against anything bigger.
Making it fill a niche like the SU-76 (good against mediums in pairs or with a Sherman, doesn't utterly dumpster LVs) would then give you some space to specialise the Jackson.
Not sure how you can make the Stuart a medium counter tho. Both the su76 and Puma out range to make up for the lack of durability. I don't see how to make the stewie able to fill the role without a huge redesign that could consume its identity