s3s packaged in Nix flakes. Add the package onto your system to make it available everywhere. Alternatively use nix run github:yusdacra/s3s-flake -- -r to run it without having to add it.
This flake also includes a module that can be used with home-manager if you use it.
Simply add the output homeManagerModule to your imports for your user and set services.s3s.enable = true. You will also want to run s3s at least once in ~/.config/s3s/ so that the config.txt is generated there (or if you already have a config file just copy it there). This path is configurable in the module options, so you can change it if you really need to. The service will run every 5 minutes by default via a timer, this can also be changed.