I started a new thread for anyone wanting to upgrade from Mellow CDY V3 to the BTT Scylla V1. The biggest issue with the Mellow board is the lack of memory, loading the latest Millennium OS you get a warning that you're running out of space and the lack of optoisolated circuits.
The board is ordered and I'll update the process when it arrives.
Here are some useful links
Team Gloomy RRF BTT Scylla
RRF Firmware Files For BTT Scylla