Without hearing you it’s difficult to say. I think a #7 should be open enough. My “guess” or suspicion would be your air supply. As we move up the horn it needs more air than the middle range otherwise it will sound as you say, thinner.

Try playing reasonably loud and as you get close to that high range increase the amount of air from down low in your diaphragm. Go as extreme as you can without breaking the notes of course and see if this starts to help.