This has been implemented, since PalmOS days, as MessageEase Keyboard. I still use it on Android. It can even be configured to allow one-stroke access to loads

This has been implemented, since PalmOS days, as MessageEase Keyboard. I still use it on Android. It can even be configured to allow one-stroke access to loads of special characters, two-step composition of accents/diacriticals (and some ligatures), and easy access to Ctrl and Alt combinations. The last bit doesn't get understood by some Android programs. The settings are a bit extensive, and several languages are supported. Unfortunately, I haven't found an open source equivalent.