I wouldn't know the first thing about implementing this myself but I'm a visual person and while I very much love using Thumbkey I would love a nicer UI to go with it. My examples might be a bit "googly" for some but are just meant as examples.
Thumb-Key is a privacy-conscious smart keyboard, made specifically for your thumbs.
It features a 3x3 grid layout, as many older phones had, and uses swipes for the less common letters. Initial testing shows that you can reach ~25 words per minute after a day of use.
Instead of relying on profit-driven, privacy-offending word and sentence prediction for accuracy, as do most popular phone keyboards like Gboard and Swiftkey, Thumb-Key uses large keys with predictable positions, to prevent your eyes from hunting and pecking for letters.
As the key positions get ingrained into your muscle memory, eventually you'll be able to appromixate the fast speeds of touch-typing, your eyes never having to leave the text edit area.
This project is a follow-up to the now unmaintained (and closed-source) MessageEase Keyboard, which is its main inspiration.
Thanks, shall do!