One thing I did notice a few weeks ago / month ago that seemed to really help was my tongue–I can’t quite explain it and will probably just confuse things if I try too hard (best just to listen to what Greg explains), but basically to me I feel like as I get up to about high C I start kind of pushing the front of my tongue down behind my lower teeth or something as I arch the back of my tongue and lower my jaw / flatten out my embouchre to prevent the aperture from closing due to my tightening the corners, etc.

This has gotten to the point where I feel much more open and relaxed when I’m playing high C and even D and E at times and am able to open the aperture a bit to get better sound and more volume, etc.