I’m not a teacher and am probably not qualified to give embouchure advice, but I’m fairly confident the problem is your setup.

I think you’re over thinking it a bit and trying too hard to prevent your bottom lip from moving.

Rolling in your bottom lip under your top lip and aiming the air down is a problem that many players do (Greg mentioned he did that, I did as well).

But, your bottom lip and jaw do move in an optimally efficient embouchure.

Your bottom lip should be inside the mouthpiece, I usually have it tucked in pretty much at the bottom.

The bottom lip must be free to move to reduce the aperture size. The top lip seems to be a bit more stationary and my understanding is it’s the primary vibrating surface.

It’s really hard to interpret things described about the inner workings of the embouchure. On one hand, we must be open to trying new things but on the other, Sound and feeling (efficiency) must guide us.

I recommend modifying your setup so that the bottom lip is inside the mouthpiece. The mmmaaaoohh thing is, for me, more of a mental guide–feel those aperture corners when you think Ooohh…

I recommend experenting with releasing a good PASSIVELY released breath (not a BLOW) through a relatively relaxed embouchure, only engagement in the outside corners and only as much as necessary. Then, when you engage the aperture corners slightly, the aperture should be reduced and the pitch will pop up.

My $.02 FWIW. Good luck–let us know how it goes.

Better yet, I recommend seeing if Greg can do a Zoom to check in on your setup / make sure you’re on the right track, as I’m pretty sure you’re not quite there.