Thank you for this (repeated) question! I will try some of these and collate my experiences.

• SwiftKey

Long-time fan, in spite of privacy concerns. My bar for comparing everything below.

• FUTO

First install, looks promising.

Indeed very customisable. What I don't like is the (imho) far inferior swipe typing and the need to explicitly switch languages for the keyboard to use the appropriate dictionary. Also, I miss directional buttons for those single-character position adjustments (Futo only offers space-key swiping). Voice typing seems highlighted but I find it to be unbearably slow.

Verdict: will most likely uninstall again.

• OpenBoard

Installation somehow defaulted to "English (Australia)", but no biggie.

Seems very customisable also, but lacks swipe typing (a deal beaker for me). Relies on the OS language (actually, keyboard) switcher and curiously lacks a shortcut to its settings (requiring the user to go so the rest through the Settings app (which, best-case, is a whopping 5 taps).

Verdict: privacy aside, cannot compete with SwiftKey for features and usability.

• Florisboard

Strainghtforward installation. Seems extremely customisable. No swiping nor autocomplete but both festures are clearly promised for a future release.

Verdict: apart from features promised in the future, thus seems an excellent keyboard.

• Heliboard

Straightforward installation. Language selection included a github redirect to manually download dictionary, which was semi nice.

Proper big-keyed numerical keyboard. Also extremely customisable. Space-key swiping even supports vertical movement.

Verdict: apart from lack of swipe typing, probably the best contender!

• Graffiti

Included because I friggin' loved it back in the day. The (to my knowledge) only app offering graffiti input is badly broken and crashes immediately on modern Android versions. I remember it working quite well on earlier versions, but that was years ago.