Your title says lemmy instance and in your description you are asking for kbin instance. Which one you truly mean?
Lemmy is already packaged in nixpkgs and its options for NixOS are: https://search.nixos.org/options?channel=unstable&show=services.lemmy.enable&from=0&size=50&sort=relevance&type=packages&query=lemmy
But I do not see kbin packaged yet.
I am using my own unstable nixpkgs, with nixmy (https://github.com/matejc/nixmy/blob/master/default.nix). Currently on 251 update of unstable channel :D I have my fork of nixpkgs: https://github.com/matejc/nixpkgs/tree/mylocal251
First of all you are probably talking about NixOS and not Nix as a server. I was using servers in cloud as well as machines at home, using Debian, Ubuntu, OpenSUSE, Fedora, Arch and even FreeBSD.
In the cloud I was using mainly two types of setup: Kubernetes and as a normal VPS and then services on top. Over the years I discovered that my favorite setup is NixOS on VPS for my services. You might ask why NixOS and not Kubernetes, does not K8s ecosystem supports all the world’s services and you might be right. But the my argument is that NixOS is way simpler, cheaper, and with less hassle, even if some services do not exist yet for NixOS, they are simple to integrate. Also my setup is reproducible with just backups of /etc/nixos/ directory and /var/lib/ both of which can exist as storage volumes. At the end, I am very happy with my setup, and do not miss conventional OS-es and other cloud solutions.
Personally I haven’t used it, but NixOS options are here: https://search.nixos.org/options?channel=unstable&from=0&size=50&sort=relevance&type=packages&query=security.apparmor