Ideally you should be able to play without your thumb even touching the neck. It’s possible, but it’s much better to have your thumb as a light anchor. The minimum pressure should be applied by both thumb and fingers when fretting.
As other people have mentioned, I think the issue is that the strings are far too high. They should be as close to the fretboard without creating any buzzing when played in your usual way.
There is nothing wrong with the actual position of your thumb, being in the centre of the neck, around midway between your index and little finger. That’s just a rough guide.
Another thing to watch for keeping your hand open so that the muscles of the palm of your hand aren’t under stress as if you’re holding something heavy and thin. You don’t want that. And keep your palm away from the neck.
And always avoid having your wrist bent while you’re playing. Not doing so will encourage carpel tunnel.