Moreover, the special extra Termux modifier keys are configurable in ~/.termux/termux.properties. I added Shift to them, because although there's a Shift key on Gboard, Shift-Enter is not treated the right way by org-mode if the Shift is from Gboard.

For some reason if you use the Thumb-key keyboard, every letter is capitalized by default, so you have to swipe down on the A before typing anything, which is a deal-breaker.