I have osteoarthritis in the thumb of my fretboard hand. Agony sometimes. I would suggest the Push brace. https://amzn.eu/d/08i7ZUtI
It is a bit pricey, but there are cheaper alternatives of the same type . I’ve found the Velpeau to be great. Plus you get spare straps included.
https://amzn.eu/d/0582tDuC
Cheers. Mike.