FUTO keyboard managed to be very similar to OpenBoard and Heliboard, while solving some issues I had with those (buggy quirks that may well be hardware or software specific).
I used Thumb-key for a few months, to give it a real go, but I'm not convinced I ever got my speed up. Could do about 25wpm at my peak but I can do another 10wpm on a more normal keyboard (with word suggestions) and I don't think I'm particularly fast. Not sure what's normal for a mobile keyboard.