Modification for cycling with one functioning hand
So I don't know much about the disease you mentioned - but if the symptoms are caused by the strain on the wrists and hands, then I have a perfect but expensive solution: Switch to a recumbent. There is no weight on the hands, ever, on all recumbent types.


Under seat steering handlebars allow the hands to rest next to the torso in a very relaxed fashion and pretty much straight, like hanging down beside the torso except the whole arms are angled somewhat towards to front.
Above seat steering handlebars probably allow for more variations on the grip, but I think the posture is overall slightly less relaxed, you can't let the arms just hang down as much.
Center steering recumbents with no handlebars (or handlebars not used for steering but only to mount brake handles) don't need any arms at all (well, if you mount coaster brakes), but more control of the rest of the body and are probably harder to ride.


I'm pretty sure all of those types also exist in three wheeled versions if the disease further progresses and makes balancing too hard.