OK, I got through it in the end.
I think the original crash I experienced mid-install, is what made things go so wrong for me.
Since I couldn’t cleanly remove the existing package as the template didn’t actually exist as a Qubes template, I cheekily cloned the debian-10 template with a name ‘debian-11’. This allowed a sudo dnf remove qubes-template-debian-11 to finally succeed.
After that, a reinstall worked fine.
Thanks for your help.