Not suggesting one way or the other but a couple things to think about. Don't know about your 2 wheel muscle memory getting confused but driving a sidecar outfit will acquire totally new muscle memories. Not much (actually hardly anything)you learned riding 2 wheelers is transferable to hacks. Point two. if you're now having upper body strength issues with a two wheeler a hack is not for you unless you're good with improving upper body strength. Not that big a deal in 1WD on pavement, even us tired old farts adapt, but a Ural in 2WD mode is a serious upper body work out. Regarding your concerns I suggest trying one out offroad before spending any money. The only learned two wheel experience that directly transfers to 3 wheelers is "situational awareness".
PS as mentioned above, if you're not willing or capable of maintaining your bike especially after off pavement use...look elsewhere.