You are correct about Greg’s aperture corners. Small changes in the aperture corners will account for a certain range. Your tongue is another factor. As you ascend the scale, the tongue will arch slightly. The tongue will flatten as you descend. The jaw is another aspect of shape. Notes below the staff may require you to lower the jaw. These shape changes work together as a system for each note. The changes are slight.

Take it slow. Do not rush. Practice harmonic slurs using crochets and increase the tempo. If you maintain a particular metronome setting and go from crochets to quavers, you will find you can’t play the quavers. I suggest you play crochets and increase the tempo gradually. You do understand it is all relative. Crochets at 144 bpm are the same as quavers at 72 bpm. Eventually, you will be able to set the metronome at 120 (or higher) and do crochet harmonic slurs for a measure. The next measure play quavers and the next measure semiquavers.