From my experience with the accordion, I would recommend making the D pad the major triad + octave, because you can only push one at a time, but it's fast and easy to spin, have the b button change the d pad to the major 4th chord, and the a button the major 5th. You can cover the whole scale that way, while still being fast and intuitive. Let the select button switch everything to the relative minor, and the start button turn on and off a simple drum machine.
I love the weeds.