That looks very cool. I wonder how hard it was to get those printed keycaps made.

Ideally you want access to everything without moving your hands off the keyboard. The DIY keyboard scene makes a lot of use of layered keymaps. It seems like the endgame for this kind of thing is not separate "keypads," but rather wider support, standardisation and adoption of multiple-layer keymaps. Half the problem is designing good maps and the other half is learning them. Sharing maps, and better software for learning/practicing the maps would help. Then you might need a few more keys to select layers. A minimum implementation might be adding a two thumb buttons below the space bar to select layers.

EDIT: inserted more text