Happy thumbs...

All the effort in a thumb goes into mantaining it's weird shape and that is what exerts pressure on the string. Thunbs have to be upright and I think easiest if kept slightly curved - think how bridges work and what an excellent weight bearing shape that is.

Thumbs nearly always have a drier sound as well. They are higher on the string, the digit pad is aimed at a different angle and the different positioning compared to the string all make a difference. You can put more thumb pad on to counteract this.

The most important bit about thumbs is keeping that useful little c shape between thumb and hand - gives your thumb space to move, stops it cramping, keeps it within normal joint range so it doesn't lock, prevents you from bringing it too far back (can do weird tension things in lower palm of hand).

Oh yeah - cut your nails...!