Hi Tina,
I was having the same issues for a while after I finished treatment - I suffer from insomnia too and my dry mouth used to wake me up nonstop. One thing that helped a lot was a facial compression wrap (usually used for face lifts), which actually was recommended for lymphodema but worked more effectively at keeping my mouth closed at night. I would think having something wrapping around under your chin may be more effective (and more comfortable) than taping your mouth since it actually supports your chin. I am sure wrapping an ace bandage around your head/chin would provide the same support - may be worth a try if you haven't already.

Another thing that actually helps me a lot too is doing my flouride trays right before I go to sleep. I have noticed that my dry mouth is consistently much worse on nights that I don't do my trays.

Lastly, rinsing my mouth with act dry mouth at night works pretty well for me (if I don't feel like doing my flouride trays some nights I will substitute the dry mouth wash). Everything works differently for different people I have noticed, but I found this mouthwash to be more effective for me than any Biotine product I tried.

My dry mouth has actually gotten a lot better in the last year, so there is still hope it will get better! I never have dry mouth "attacks" anymore, and I often even sleep the night through without having to wake up for water. Of course, my mouth is always very dry when I get up so I am not sure if part of my sleeping through the night is attributable to me just getting used to it, but either way dry mouth no longer keeps me up at night so that's all that matters

I hope this helps!