Its my turn to help you Tim
One reason why your board slip below you or you are landing on your back is that as you said to much rotation.
in order to avoid this , while you approching the wake bend your knees and be real patient wait wait wait until you feel off the wake.
One key while doing this is that do not through your head back whil doing the trick, the wake and good approch will make you rotate. If you approach is corerct and you through your head back it will accelerate the rotation. Try to feel like you want make an indy grab at the same time, be in air first and rotate only after.
An other thing that make your speed in rotation is in case you bend your knees while rotating. More straight your legs will be slower the rotation will be.
hope it help