I think it is abundantly clear that no one solution will satisfy everyone, let alone be ideal for everyone. Hence: lots of configurability!
I can understand that. I’m a bit used to it because that’s what LibreOffice does (auto-completion inline). Probably for a keyboard, I would prefer no auto-completion inline and “three options on a bar”, as you say, because, in my case, that’s what I am used to from the iPhone.
However we are muddying the waters and talking about a bunch of different functionality:
- auto-correct
- auto-completion inline
- suggested completions out of line
- auto-spellcheck
plus, unrelated, alternative character popover
plus questions about what dictionary or dictionaries get used for the first set of functionality.
Auto-spellcheck is useful because that is an easy way of adding “new” words to a local dictionary, which then feeds back into the other functionality in the first set of functionality.