And regarding strengthening the thumb: my thumb usually sits quite loosely on the neck, sometimes it even lifts off, or only the tip of the thumb touches the neck. Not sure how this would strengthen the thumb. So perhaps this is also part of my problem!?
Pain is a warning that something is wrong with your technique. I gave myself tennis elbow for a year because of over practicing barre chords. For me, the treatment was Trigger Point Therapy.
I decided that I needed an exercise to work on fretting hand ring and little finger independence. So I devised a simple exercise wherein I tap the little finger using very small taps that keep it in one place, while moving the ring finger back and forth. Then swap the fingers.
Surprisingly mentally harder than expected!
And, apparently, physically harder than expected. A short 10 or so minutes slowly figuring out how to move the fingers like this and bam! Tendinitis at the wrist. Ouch! Hard to play right now and I can only blame myself.