I have struggled with a sore thumb for years. I certainly agree with Tomas on Positioning.
There’s also the pressure concern don’t use more pressure than necessary.
Niether of these concerns fix it for me. While I certainly try to adhere to both,
it doesn’t seem to help much. What does help is, I take breaks in my practice time
or divide it into sections. I do warm up exercises BUT DON’T OVER DO IT.
Thirdly, I exercise my thumb durring the day. I simply move my thumb in circles.
I find it a little painful but doing this relaxes my thumb and makes it easier the next day.
Hope this helps