Thank you @nkatakis and @njoseph for your helpful answers.
I guess I took “Debian” to mean “debian-based”, ie Ubuntu would suffice. In the context of your replies, I see now that “pure blend” means "not Ubuntu or anything else except pure Debian.
With the benefit of hindsight I’d hope this point is clarified in the install instructions/wiki. (I’d edit it myself if I already had an account on the debian wiki).
If you want to install it I would recommend to install one of the images you can find on the website for your specific hardware.
In my case, I’m installing it as a container within Proxmox, so yes, easy to change to Debian, it would be nice to have a container template with Freedombox already installed. Perhaps I’ll look at making one.
Thanks again