XFCE has it's place, well deserved.
You can use Linux Mint Debian Edition 6, it works better than any Ubuntu after 10.04, and better than any Debian and that is really strange, but hey, it works. I used it for roughly 6 months on several PC and did not have Debian issues (disappearing stuff from panels, stuff crashing, again rarely), except once.
Installing LinuxCNC is the now standard set of two lines:
sudo apt update
sudo apt install linuxcnc-uspace
You can also use any version of Mint, but LinuxCNC must be built from sources.
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Did you try installing a Debian RT kernel?
That usually works, as simple as downloading and installing using gdebi. Try this
ftp.debian.org/debian/pool/main/l/linux-..._6.10.12-1_amd64.deb 
