I think electric(/-assist) is definitely worth looking into. Try and see if you can find a local shop that has a loaner program, or maybe rent by the day, with your rental applied to a purchase if you pull the trigger.

A more unconventional suggestion: pop over to the Adaptive subforum, and ask there about handcycles. Normally for amputees, but if you can't use your legs, maybe this is an option. Also recumbent fans might be able to advise whether that could be expected to provide knee relief.