My grandad before he passed away from surgical complications had Parkinsons and loved to play guitar! It was his primary passion in life outside of tinkering on whatever gizmos had captured his fascination at the time. He pretty much switched from picking to strumming when he couldn't pick anymore, and when he couldn't hold his guitar on his own he'd sit back in a recliner so that the guitar resting against him a bit more, and eventually some extra straps and trays to help out balance and adjust posture and such to hold things up and in place. My general advice is find what works best for you and modify if you need to. It can be really frustrating to feel like you can't play as well as you used to, but it's okay if you miss bits here and there. I may not have Parkinsons myself and know exactly those struggles, but I do have visual problems that need constant adjusting in order to keep doing some of the things I love, and there's going to come a time when I can't do them anymore because my vision has deteriorated too far - so I can fully sympathize with the feelings expressed here.

Cheers,
Ember