After using org-mode since beginning of pandemic, I've realised that I do little to no editing on my phone.

But for that little editing, Orgzly all from f-droid is pretty great. As a side bonus, it handles TODOs from my org-agenda to generate Android notifications! Very handy and very private.

I'm not sure of Syncthing story on iOS as I don't have an apple device, but you can always store your notes on dropbox/icloud/whathaveyou. Unfortunately I lack any experience to be helpful with Apple devices otherwise.