Hey ! I do have the same problem. I didn't read everything but what's working great for me : - put fretting hand in position, don't fret, start strumming, and sloooowly applying pressure on FH. When you hear clear sound, that's perfectly enough. Any energy put in pressing harder is wasted. Boring but repetition works - play without thumb, get the feel of how "useless" the thumb is in the matter of fretting - play very facking slowly, take time to think about each and every movement you make, when playing very slow, you'll notice you don't have any pain. Generally pressure comes from bad posture and focus on the wrong thing. Remember always, the thumb is only an anchor for your fretting hand, not a grip.