Thanks for linking to the videos! Looks to me, though, as if his wrist is really straight only when he plays open chords. And then his thumb is way above/around the neck. When he plays barré chords, his wrist is still a bit bent (and I would really like to see where his thumb is then). So isn‘t this what I mentioned before already? Though I have to admit that my wrist is probably much more bent than his, and I’m not even playing barré chords
And regarding strengthening the thumb: my thumb usually sits quite loosely on the neck, sometimes it even lifts off, or only the tip of the thumb touches the neck. Not sure how this would strengthen the thumb. So perhaps this is also part of my problem!?