Heh, I didn't feel very comfortable yesterday trying to get this working. Don't worry, well get you multi material printing. Once Prusa ships my MMU2S I'll have first hand experience with it, right now it's somewhat of a guesswork how everything works in it.

I managed to get RRF working. Once I established a serial connection to the board using screen in linux (screen /dev/ttyACM0 115200) I saw that the board was resetting itself every time I tried to load the web UI. I tried a bit older release and it worked a tad bit better, even managed to get some text loaded on the web UI, but it kept resetting still.
Finally I noticed on the config.g that I had enabled the 12864 display I was using with Marlin and tried disabling it as it was showing garbage anyways and you have to define your own menu apparently (need to investigate that more in depth later). The display part was suggested by the online configurator (which is a must if you want to configure your printer): https://jaysuk.github.io/LPCConfigurator/Start. After disabling the display, it just started working. Web UI works and printer seems stable. I tried printing benchy, but the quality was awful due to the print cooling fan not working properly, but a easier stackable box print turned out nicely.

Needed to do M303 heater tuning for bed and nozzle, bed heating was giving me errors about the temp not rising fast enough. After the tuning heating seems to work fine.

So far looking promising although took a bit more tinkering to get at this point. The print quality seems at least comparable to Marlin, but need to do proper print comparison. Luckily swapping between Marlin and RRF is easy, just replace firmware.bin on sdcard and off you go. Actually with Duet web UI working when RRF is flashed I can upload the Marlin fw through it if I don't want remove the sdcard from the printer. Getting back to RRF requires removal of course as the DWC isn supported by Marlin.

Things to address:
- print cooling fan. By default RRF sets the PWM to 500hz and apparently that's not a good frequency for at least this particular fan, so that needs to be tuned. The fan works @100% but with lower speeds (75%) it's stopped
- Pressure Advance (RRF's version of Linear Advance) tuning, currently using 0.28 as with Marlin but I suspect those values aren't 100% interchangeable. But at least it seems to be close enough for now
- check how the display should work. Not a big loss if I can get it working, with DWC I can use phone or tablet to control the printer.
- see how manual mesh bed leveling is done with RRF. With Marlin it's easy when you have a display, hoping that with RRF the web UI provides help with that. Otherwise it's going to be tedious feeding the gcode commands in console.