I also do notice that the balls that I have drilled with a reverse thumb pitch seem more difficult for me to get into a full backswing because I unknowingly am afraid that the ball is going to drop off my hand.
Bingo! that is what I have been talking about, now you see how much you squeeze it and didn't realize it before. Now watch how free your swing gets.
As far as bending from the waist to increase your back swing height goes, make sure you let the ball fall into the swing and the ball starts heading up into the back swing before you bend, otherwise you will cut the swing off again and swing the ball with your back instead of your shoulder.
Try just letting the ball swing free without the excess back tilt before you try going higher, it is best you learn to swing the ball before adding any moving parts.
If you just let the ball swing from your shoulder, you will automatically tilt your spine the proper amount without thinking about it unless your body has an odd center of gravity that causes you to bend forward too far, then you might have to force yourself to walk more upright. If you are thinking that you are already too far upright, you are probably just fine and shouldn't mess with that at this time.