Atom scrapper is in a separate binary (rostra-bot), the --help there should help you configure it.

But in my NixOS config I configure it like this:

  systemd.services.rostra-bot-rss = {
    wantedBy = [ "multi-user.target" ];
    serviceConfig = {
      User = "rostra-bot-rss";
      Group = "rostra-bot-rss";
      StateDirectory = "rostra-bot-rss";
      # TODO: add more feeds
      ExecStart = ''
        ${pkgs.rostra}/bin/rostra-bot \
          --atom-feed-url https://dpc.pw/atom.xml \
          --secret-file=/run/secrets/rostra-bot-rss \
          --data-dir=''${STATE_DIRECTORY}
      '';
      Restart = "always";
      RestartSec = "5s";
      LimitNOFILE = "100000";
    };
    environment = {
      RUST_LOG = "rostra=debug,info,iroh=error,mainline=error";
    };
  };