expansive repositories

That would be new for me. AFAIK Debian doesn’t have that many packages (compared to AUR or even nixpkgs (see https://repology.org/)). Regarding Flatpak: What packages do you need for a server with Flatpak? Desktop makes sense for me, but I haven’t yet had any use-case/package for server related software in Flatpak.

I switched from Debian to NixOS for servers, 3 years ago, as I think it’s easier to maintain long-term (after being on Debian on servers for years). A new install (after EOL Debian support) often is a little bit more hassle and requires a longer downtime in my experience (apart from the lack of reproducibility and declarativeness and the sheer amount of software packaged and configured in nixpkgs).