There is a lemmy nixos module https://search.nixos.org/options?channel=unstable&from=0&size=50&sort=relevance&type=packages&query=lemmy in nixpkgs which is functional. Or at least almost there in terms of ease of use. Even with experience, it still took me over an hour before I could successfully start my instance.
ofc this doesn't cover the host creation, and other details, I already had a server with NixOS running.
Is there something wrong with the nixpkgs module?
Nothing at all. Maybe my suggestion is ignorant (but I don't think it is).
What I am suggesting is a template configuration for instance operators to make getting up and running fool-proof. From what I hear, the nixpkgs module is great but it doesn't help with the more complex parts of spinning up an instance regarding DNS and other aspects of a typical instance's tech stack. If we could share a github repo with a FULL minimum viable product for all of the parts needed to run a real, solid lemmy instance, it would allow instance operators to gain all of the advantages that we currently enjoy using flakes. For example, an instance operator might be able to safely upgrade their entire instance with one command while, from what I observe, it currently takes instance operators a week or more to test and install things in that old, painful non-Nixy way.
It is possible to deploy a lemmy server via a NixOS option. It's under services.lemmy, see https://search.nixos.org/options?channel=unstable&from=0&size=50&sort=relevance&type=packages&query=lemmy
You could always try to improve the packaging (there is an issue I had, but a ticket was already open and should be fixed at worst in the next version of lemmy, which was released a few days ago if I'm right)
Of course, everything that make NixOS server hosting will making hosting lemmy with nixos easier, but most of the trouble I had with setting up my server a few years ago was understanding how to configure and run the DNS server on it.