Thank you! I'll try playing with out the thumb and see how it goes. And yes, I've found that pointing the thumb toward the headstock relieve some of the pain. I appreciate it.
It seems you already have an answer but I'll weigh in just because my personal experience is similar. I have osteo-arthritis in the thumb joint on both hands. For a couple years I was using a hand brace that helped but didn't totally alleviate the pain after gigs.
Medically I was recommended cortisone shots and possible surgery but I didn't want to go that route if possible. Or at least put them off for now anyway. I did alot of research and really tried to investigate my playing style. After all of that I did the following.
-started pick playing which IMO greatly increased note clarity with less fretting hand pressure
-learned how to do setups correctly myself and got the action on my basses as low as possible
-I already play only SS's so that wasn't something I needed to change
Based on a recommendation from another player I started practicing by not using my fretting thumb at all. Any "pressure" needed against the neck comes from the point in my hand where the finger knuckles hit the neck. And after doing that for awhile and getting it down, if my thumb is on the neck it's pointed at the headstock about 80% of the time, but I still fret E string notes from time to time with it too.
If it's helpful here's some photos of what I'm talking about that I posted on a different thread.
Point where the "pressure" on the neck comes from when not using my left/fretting thumb.
How I hold the thumb when not using it. And I'll find that sometimes at gigs I'll feel a little pain creeping up and realize again that I'm pinching the neck. It just happens when you're in the zone and thinking of other things. So at that point I'll pull my thumb away and things clear up again. No more brace needed and no more pain.
Just my 2 cents, hope it's helpful. This getting old thing hits us differently if we're lucky enough to still be here. Best of luck on your progress.