Steve, if your “arduino-foo” is strong, then you can try configuring Marlin yourself. The RC8 has had a problem, not exactly sure where, but it has to do with the configration of the MPCNC, and the specific RC8 version. RC8Bugfix seemed to fix the issue, as did RC7.
If your “arduino-foo” is not strong, or you maybe want to make sure everything else works first, start with the configured version of RC7, at least until you get a test project done. It would suck to have to determine if it’s an RC8 issue, or somethings binding, etc. The RC7 has a lot of hours logged so far.