I used to have just that ”thumb cello hold”, but it’s much better now. I practiced without using the thumb and tried to feel the weight of my hand pressing the strings. It was great method to loosen the thumb, but it worked only little time at first. Then I found myself squeezing it again. My only solution to this problem has been time and practicing more. One thing I have noticed is that when I’m playing some new and difficult piece, my thumb easily stiffens again. When I learn the piece, it’s easier to keep it in the right position.