I had to go pick up my bass to see how I play LOL
I've also got the hitchhiker thumb, does makes thumping easier

. My left hand pivots off the fleshy part of my thumb (side opposite the nail), which keeps my joint straighter. If I pivot off the joint, the thumb is in the bent position and gets tired/sore fast. Try keeping your hand in more of a "c" position and see if that helps. I always envied how my teachers hands are so flat and it never even looks like he's fretting anything, but my hands just aren't comfortable like that.
I do find that I keep my thumb anchored towards the top edge of the neck (closest to my head) versus in the middle of the neck, which also helps with the angle. I play a 5 string and can still reach all the strings from there (and I don't have big hands). The only times I have trouble with the one finger per fret are when I'm low on the neck where the fret spacing is, my third finger won't reach. I cheat and use my pinky because it's in the right place without stretching my hand. Use whatever's comfortable for you!
My dad has gout pretty bad and found out that what he eats affects how his joints feel-you may want to examine your diet. He avoids processed meats, anything with msg, anything malted (including beer and wine), pickles, sausages, anything high in acid (tomato products, vinegar, etc), keeps salt low, etc. It keeps his gout and arthritis from flaring up badly.
I notice that if I eat anything containing any kind of artificial sweetner I have more trouble with my knees hurting/carpal tunnel flaring up (not to mention headaches and stomach issues). It seems *everything* has artifical sweetener-try finding a non sugarless gum! Even my toothpaste had it (switched to a "natural" one). Might be something to look into........