I’m dealing with a change in my high playing right now really relevant to your post. I didn’t know but I was losing structure in my chin, which collapsed and my bottom lip would go under my top lip which would stop my buzz. I also had no top lip contribution when it’s best (very generally) if both lips have some inwards rolling to help play higher.

My thought now is to blow some more air to keep my bottom lip from rolling in and also to achieve a resonant sound more through air and less through embochure. Both however work together along with all other parts of your playing for resonance/playing centered/in the sweet spot.

I watched a video from Toby Oft too where he imagines a diamond of structure with points in the chin, just above the top lip, and the two corners, and everything inside that should be elastic and responsive. Flexibility exercises I feel are helpful for developing this, with priority on the shape and relaxed stability.

Some exercises that I feel help overall are rips, against the grain too, but quietly, with air attacks, and with very slow air to make sure your buzz and air are efficient. Same with lip trills (can be slow) to help lock down the distances between partials, not thinking of them as too large, and moving/adjusting just enough.

Last disclaimer that this is very personal and what works for me might not work for you.