I used to focus on the items you mentioned, but lately I’ve focused more on lip strength and air speed. The lips have to be firm enough to keep the pitch up and soft enough to vibrate.

Air needs to be more focused on air speed than volume of air.

Try taking out the tuning slide and playing on the horn. Pitch should be a concert Eb. Too low? Focus on air speed and direction. To high? Relax the center.

If you’re having students make sound by “buzzing”, switch to having them let the air make the lips vibrate. Similar result but completely different concept and a better starting point for good brass tone. I don’t even use the word buzzing anymore.