nixos

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.

Lemmy is packaged already, and you can find configuration options here.

kbin is not packaged yet, thus there are no configuration options for it.

There currently is an open issue requesting it to be packaged here.

Will the steps on https://codeberg.org/Kbin/kbin-core/wiki#admin-guide (but replacing the apt installs with adding packages in configuration.nix) just work

No, they will not. It's not about having packages available, it's about packaging it properly and configuring the system to accommodate it.

or is there a better, more reproducible way?

First you'll need to package kbin. This might actually be rather trivial since php does not involve a real build process AFAIK? See other PHP packages for inspiration.

Next you need to set up a module that internally declares a service and configures other services such as Redis or Nginx accordingly. Again, see other PHP modules for reference.

An alternative would be to use Docker/OCI containers to run kbin.