Getting Ashes 2063 to run was a bit of a challenge, turns out the GZDoom snap package is dogshit and won't run anything, but the .deb version from the GZDoom website works fine.
I wasn't sure if the Doom 2 WAD was strictly necessary to make it work, since FreeDoom should fill in the gaps, but Internet Archive had a copy of the original WAD so I got it anyway.
Upshot is, everything works now so Ashes 2063 is still on for next Saturday.