Alright, well, I have a Drupal site now.
It's unavailable at the moment; I still have to set everything up.
The setup I finally got to work uses Nginx for the reverse proxy/web server and PostgreSQL for the database, both of which are set up in the traditional NixOS way. The Drupal site itself is not, though; I finally just gave up and made a directory for it, and installed it to said directory using Composer, manually.
So the site itself is the only part that isn't "reproducible" according to NixOS. I manage the site myself; it's not actually reproducible, to me, when NixOS manages it for me, because it does some kind of black magic involving symlinking directories outside of the Nix store, to the Nix store. Not all of it, though. So I didn't actually have direct access to the entire directory for the site, because that was a Nix store path, which could change at any time. This is one of those few times when I got tired of fighting NixOS and just did something normally. 