Heliboard isn't perfect but is an openboard fork that has spell check, is getting active development, and is on F-droid. There's even a correction sensitivity setting. It's the first time I've been able to move away from Swiftkey.
I've been using openboard for a while now and was pretty annoyed at the lack of multilingual typing, to the point that I turned off autocorrect. Thanks for making me aware of Heliboard!
"We’ve seen cases where it doesn’t work on Samsung devices. The reason why is that Samsung changes the spell-checking API that Android provides, making it impossible to actually do any check"
Fuck I got a Samsung. I'll have a better look later.
I thought this was app specific, not the input keyboard. I'll be following to know. The keyboard will give suggestions at the moment of typing that specific word, which isn't great for trying to go fast.
I think Grammerly does this, but I'm not sure if it's universal or if it's specifically inside it's own app (just seen ads haven't used it myself). What you want is a "spell checker" instead of "autocorrect," though, which might help the search.
In all the keyboards you used did you set the language and turned on autocorrect with additional features such as auto punctuation and all.
Samsung keyboard works for me better then others.