Years ago working in the kitchen I splashed melted sugar (ooh, creme caramel…yum) all over my right thumb. I installed a thumbshifter for part of the summer to get me through race season. This might work for you.
It is progressive…I never had that until maybe a couple months in without trying to resolve it. Severely painful and usually in the morning after sleeping. It would loosen through the day to where it was semi-usable.
Are you 100% sure it’s caused by shifting gears? I used to get a bit of thumb pain from riding. For me it turned out to be my brake lever angle. I tipped them up (a bit closer to flat) and my thumb pain went away. Have you already tried that?
I should also add that at the time I was braking with two fingers. I cut that habit and forced myself to only use my index finger at the same time as adjusting my brake levers. That might have reduced the thumb pain as well.
Years ago, I went over the bars while trying to brake for another rider who didn’t know WTF they were doing, and landed on the tip of my left index finger. It swelled up like crazy and never really went down, causing major one finger braking issues. That was, until I started playing guitar again and it started to get lots of little micro stretches. After a few months of that, my finger was back to normal with full range of motion.
I’d suggest finding a stretching routine and begin icing it on the regular (i’m horrible at the latter), especially after riding.