I’ve noticed that thin necks cause me to do strange things with my thumb on the fretting hand, leading to discomfort.
What has worked for me is to try playing something without using the thumb and then looking at how my hand is positioned on the neck. Then you can place the thumb back again without putting pressure on it, because you already got used to playing something using only your “fretting strength” and not by gripping the neck
I did a technique course with Scottsbasslessons and he had us do an exercise where we played without placing our thumb on the back of the neck to get used to the minimum pressure required to get the string to sound cleanly. You'd never play regularly like that but some simple scales every day with no thumb significantly reduced the 'death grip' I used to have on the neck.