Hi. Great video. I’m not really in a position to comment as you are a far more experienced player than me. However I think your only problem is you pucker far too much.

IMHO if you just back off the pucker I think the buzz will come easily. How do I know because initially I did what you are doing. I pushed the lips too far forward into a pucker and I could not play. All I had to do was to backed off the pucker then placed the mouthpiece on the lips and bingo. Not sure where the round opening comes from. For me as I ascend the firming of the aperture corners just makes my oblong hole between the lips smaller in width and height. I also feel the tension of the centre of the lips increase slightly. I think of this as a guitar string tension increases so pitch increases. Combine this with tongue arch and you have the whole shape concept. These are just things I feel and you may well feel something completely different so take my comments with a pinch of salt. The pucker though I think is far too much. Hope this helps.