I have not looked at the 3DPCB-Keyboard until you mentioned it.
The setup is correct. but the layout not.
I will start with the 2 row of buttons first. to see if I can see the full potential. the buttons on the right (of the Micro) I will try to replace a slide resistor as a t-bar. Or rotate the Micro to put a 45mm slider blow it.
I have almost all components available except for the RGB led strip. I’m not sure what would be the best next step. go with the design you made. or take it as a baseline. and use a higher density of RGB leds and put more buttons next to each other. with a 2 button row.
link to the functionality:
Here are a few links of controllers that people made. But your design is sleek and simple.