I've updated the base library at https://dboucher.org.uk/keyboard/Thumb_Keyboard.zip to the current Arduino and PlatformIO standards, so you can now install it in Arduino using the "Add .ZIP Library" option or in PlatformIO using:

platformio lib install https://dboucher.org.uk/keyboard/Thumb_Keyboard.zip

My next task is to do the same for the 23017 version of the library and move the key label and 3D printed part files into the "extras" directory of the base library to finish tidying things up.

EDIT: All done.