Turns out the code/file required fewer files than I was expecting but that's mainly down to using the Blackpill Module.

The keyboard now loads in usevia.app and I can record macros...

Although the keyboard generates the correct key presses, the via app shows the key legends shifted one place to the left after the encoder position. Also issues with RGB and Encoder code as when I try to add any, I get compile errors... but there's some progress :) ...

Life would be so much simpler with more GPIO's, a 6x17 matrix requires less work in code...