Hello Miguel,
I should say up-front: I’m by no means an expert, so my answer may not be accepted wisdom, but your question rings a very loud bell with me. One of my recent “discoveries” when armed with Greg’s magnifying glass may just help you, too.

I have been experimenting, watching and, frankly, pulling my hair out for some time wondering about shape, aperture corners, pinching, clamping. What has really helped me is not thinking about tension in the aperture corners, but looseness at the centre of the lips. I know it’s mentioned in various places, including in this exchange, but I don’t think it is really stressed enough. You have to (somehow!) tighten the corners to change pitch while at the same time NOT tightening the centre of the lips. If you don’t leave the centre of the lips loose, you need to blow an awful lot harder to get them to vibrate, and all that backpressure and choking starts. One of Greg’s exercises I’ve found really helpful is putting two fingers between your teeth and then doing the “aah ooh”. (Sorry, can’t remember which video it is.) You should feel the corners of your lips tighten, but you should not feel the centre of the lips pushing up/down on your fingers.

I hope this helps, but please don’t take my word as gospel. As I said, I’m only a fellow traveller, not a tour guide!

Good luck,

Jutta