@timgibby It is actually a very good question. I'm following this topic to learn more :). I for e.g. recently made one more (miniature 3 axis, 2A motors) CNC here and was thinking on what to use for control, considered a lot of options as I do have a bunch of hardware available. I decided to take the Smoothieboard from one of the 3D printers running smoothie and use Smothieware in GRBL mode with RepRapDiscount smart Graphics display for the CNC and pushed Duet2Ethernet with paneldue into that 3D printer.
I love how DRO looks on Smoothieware, you can see WCS and MCS values, all the NIST stuff works... now, I think all this works with Duet too but in my case I currently, for the 3D printing, prefer RRF to Smoothieware, so I upgraded printer to RRF and used Smoothie for the CNC. If you already have RRF capable board you should try, but as mentioned, if the current capability of the RRF board is satisfactory. Using RRF board with 5-6 drivers and not using those drivers is kinda expensive :).
Dunno if GRBL postprocessors can be used with RRF or separate postprocessor need to be used?