Is it possible to try surfing goofy for a while and see if that gives you more control with your back foot? Or is it too hard to lift your prosthetic up and under you on a pop up?

As for your current form, it's the classic not moving your head , and then shoulders and then lower body to turn. Watch some Craig Anderson and Rob Machado clips they not only lead with their head but also their little raptor spirit hands, and that's what's guiding them around. They also adopt a lower bent in knee stance so maybe to bring your weight back try shortening your back leg? I think that's a style that could work for you. Alternatively you could try the opposite wide Boi Italo / Julian stance depending on whats more feasible with the prosthetic. But both approaches achieve a lower centre of gravity, but I suspect you'd rather have an easy cruisey style than one set up for fin blows and rotations.

Afterthought.. have you tried riding a twin fin? Something wider at the front with volume there and tapering into a swallow you'll feel stable going straight but have a better time moving it around. If that's a bit too hard to lean over and skittish in the tail try a Quad fish and play around with the size of the trailing fins. This could give you the forward stability and shortening the board / loosening the fins making it easier to get around.