TLDR: I could run the service as my user manually but it wouldn’t fire properly inside a service that would intialize upon system boot.

TLL2R (too long love 2 read): I first had to install steamcmd and steam-run into my system by modifying the /etc/nixos/configuration.nix file to pull the packages from the nixpkgs repo and build them into the system. The primary and really, only way, to interact with packages and install and alter the /root directory is to edit this file and run the rebuild.

Next, I had to recreate this systemd service to work inside nix:

Original, linux-agnostic systemd service configuration file per Satisfactory Wiki:

Description=Satisfactory dedicated server
Wants=network-online.target
After=syslog.target network.target nss-lookup.target network-online.target

[Service]
Environment="LD_LIBRARY_PATH=./linux64"
ExecStartPre=/usr/games/steamcmd +force_install_dir "/home/your_user/SatisfactoryDedicatedServer" +login anonymous +app_update 1690800 validate +quit
ExecStart=/home/your_user/SatisfactoryDedicatedServer/FactoryServer.sh
User=your_user
Group=your_user
StandardOutput=journal
Restart=on-failure
WorkingDirectory=/home/your_user

[Install]
WantedBy=multi-user.target

My final modification to make it functional inside Nix:

      enable = true;
      after = [ "network.target" ];
      wantedBy = [ "multi-user.target" ];
      serviceConfig = {
          Restart = "always";
          ExecStart = "${pkgs.steam-run}/bin/steam-run ~/Satisfactory/FactoryServer.sh -unattended";
          WorkingDirectory = "~";
          User = "me";
          };
      };
  programs.nix-ld = {
          enable = true;
          libraries = with pkgs; [
              steamcmd
              steam-run
            ];
};

It wouldn’t work at all at first because it seemed to fire and run with SUCCESS by running systemctl status satisfactory but I couldn’t connect to the server.

Then, I realized it WORKED for real when I ran the $ steam-run ./FactoryServer.sh as my logged in user, but not as an automatic service when the system booted.

I believe it’s because I had /run/current-system/sw/bin/steam-run instead of the ACTUAL hardlinked file. There is probably a way to make it run properly that way, but it also seems as dubious a reason such as: the systemd service probably can’t access /current-system/ in the right way or at the right time.

Further detail:

running steam-run from my user uses this one: /run/current-system/sw/bin/steam-run

which is just hard linked to the real source: /nix/store/gigaxzirmbs51c2cab1ddyg2x4x258py-steam-run/bin/steam-run

changing the systemd service configuration in /etc/nixos/configuration.nix to instead run the start up script with "${pkgs.steam-run}/bin/steam-run fixed it.

Further work to be done:

Need to figure out how to get ExecPreStart working so I can update the server when it starts rather than having to manually stop it and update it.