How is it not a light abstraction? If you're familiar with systemd, you can easily understand what the snippet below is doing even if you know nothing about Nix.

    systemd.services.rclone-photos-sync = {
      serviceConfig.Type = "oneshot";
      path = [ pkgs.rclone ];
      script = ''
        rclone \
          --config ${config.sops.secrets."rclone.conf".path} \
          --bwlimit 20M --transfers 16 \
          sync /mnt/photos/originals/ photos:
      '';
      unitConfig = {
        RequiresMountsFor = "/mnt/photos";
      };
    };
    systemd.timers.rclone-photos-sync = {
      timerConfig = {
        # Every 2 hours.
        OnCalendar = "00/2:00:00";
        # 5 minute jitter.
        RandomizedDelaySec = "5m";
        # Last run is persisted across reboots.
        Persistent = true;
        Unit = "rclone-photos-sync.service";
      };
      partOf = [ "rclone-photos-sync.service" ];
      wantedBy = [ "timers.target" ];
    };

Hah I just wrote something similar today to periodically push backups to another server from my NAS.

I agree the systemd interface is rather simple (just translate nix expression to config file). But NixOS is a behemoth; Completely change the way how every package is built, introduce a functional programming language and filesystem standard to somehow merge everything together, and then declare approximately every package to ever exist in this new language + add a boatloat of extra utilities and infra.