There’s some good advice already mentioned here.
What I would watch out for with your thumb is making too much of a circular motion when bringing your thumb back up to play the next note. This is usually when your thumb pick hits the head, it’s when your coming back up after playing a note.
Try to work on bringing your thumb straight up after playing a note.
But if you can record a video that’d be great and I can take a look and comment further. Thanks!