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This is the board that https://www.circuits.io/circuits/4592 plugs in to.
It looks like the best idea is to embed an Atmega328p as Arduino compatible running GRBL, with USB connectivity, and jumpers for microstepping selection. I had really hoped for at least one rotation axis, but GRBL only supports X,Y,Z. I am still pondering maybe having a 90 degree female header to replace Motor3, header being specifically to interface with the Raspberry Pi GPIO header. The boards would all stand vertically, and the RPi could talk to the GRBL system directly.
Currently, I have an Atmega328 28pin DIP connected as GRBL would have had an Arduino connected. Power and additional processor support are still needed.