FUTO Keyboard is the best combination of everything.
spell checker
local on device voice-to-text
swipe
You can add their F-Droid repo, or get it through Zapstore
Discussion
I can't find it in the F-droid repo. What is the exact name or F-droid link?
It's not in fdroid.
I used to use gboard with network permissions off.
I've now gotten Futo's settings to the point where it's a similar experience.
It took a little while.
Get the best FOSS Android keyboard that had swipe and privacy included at:
It's in alpha, so you can expect bugs and crashes, although I haven't experienced any yet after using it for a few days.