The micro controller is an Adafruit Feather ESP32. This board has the advantage of being very small.
The Stepper Motor Drivers
I designed an built a custom stepper motor "shield/wing" to drive the stepper motors. See the files section for the gerber files if you want to have your own printed. The board holds up to 4 Pololu DRV8835 motor drivers.
These can control 1 stepper motor each (not current limited) or 2 DC motors each. For a total of 4 stepper motors or 8 DC brushed motors.
The Stepper Motors
The stepper motors are Pololu "1204" Nema 8 stepper motors, but any Nema 8 sized stepper motor with an appropriate length shaft and voltage rating should do.
The motors have 3D printed face plates that let them interface with the LEGO pin holes.
Control is with a touch base onscreen joystick. I use the X/Y to skid-steer algorithm from here: http://home.kendra.com/mauser/joystick.html
The ESP32 then uses hardware interrupt timers to precisely control its motors.