The club flattens out naturally if the down swing is initiated by the lower body. Try breaking your knees to the target and almost feel like you are falling back. Moe Norman demonstrated this. It’s hard to break the habit of feeling like the shoulders are going to drive the swing.
Breaking the old swing habits are THE most difficult things to do. Is there a video depicting the lower body move in order to try to work on this? I am a 100% upper body/arms initiator of the swing, I know it/feel it but can’t break it. Had a short period of time where I broke the habit, swing seemed effortless but fell right back into old habits.