The main show-stopper for me regarding writing long documents on IOS has always been apparent lack of a keyboard command for moving by paragraph. A Google shows reference to this shortcut for IOS Word, but is it available system-wide in apps like IA Writer and Ulysses, Notes, etc? I've gone through the dance of downloading, running, being completely frustrated by, and uninstalling various text-editing apps multiple times on IPhone.
The advent of IPad OS has me thinking about getting an IPad, if I can finally do some productivity stuff with it, at least with a keyboard.
Navigation by heading is limited to Apple Pages, I guess, and that seems unlikely to change.