I have 2 Duet boards, Duet2Wifi and Duet Maestro. Both use RepRapFirmware https://github.com/dc42/RepRapFirmware and I don't think Marlin is available for them. But the hw is very good quality. Also Duet is very flexible in sw since everything is configured with gcode and macros so no need to compile fw.
There is some discussion about MMU2 with Duet boards here: https://forum.duet3d.com/topic/6155/...-s-mmu-v2-duet and it seems someone has got MMU2 working, at least somewhat (did not read all of the thread...).