Hello and welcome! Definitely landed in the right spot here.
I’m not sure how many other people here have explored the idea of converting Flowboards to electric, hopefully more than a few.
I didn’t come up with any better solutions than you have for getting the regular wheels to be driven. My idea was pretty in line with your centered pneumatic drive wheel. Except I was going to try slapping on the suspension powered Castor from E-Freeboard Open source Electric Caster project.
I’m not sure if you felt like having the extra center driven wheel made the ride much worse than without it, but I was hoping a suspension caster would basically make it feel exactly as it does normally.