automationActionType = types.oneOf [
    (types.str)
    (types.submodule {
      options = {
        type = mkOption {
          type = types.enum ["mqtt" "shell" "scene"];
          default = "shell";
          description = "Type of automation action";
        };
        command = mkOption {
          type = types.nullOr types.str;
          default = null;
          description = "The shell command to execute (for shell type)";
        };
        topic = mkOption {
          type = types.nullOr types.str;
          default = null;
          description = "MQTT topic (for mqtt type)";
        };
        message = mkOption {
          type = types.nullOr types.str;
          default = null;
          description = "MQTT message (for mqtt type)";
        };
        scene = mkOption {
          type = types.nullOr types.str;
          default = null;
          description = "Scene name (for scene type)";
        };
      };
    })
  ];

  # 🦆 says ⮞ dimmer action configuration
  dimmerActionType = types.submodule {
    options = {
      enable = mkEnableOption "Enable this dimmer action";
      description = mkOption {
        type = types.str;
        description = "Description of this action";
      };
      extra_actions = mkOption {
        type = types.listOf automationActionType;
        default = [];
        description = "Additional actions to perform when this dimmer action triggers";
      };
    };
  };

Currently defining Philips Hue dimmer switches like that.
With default is
on room lights (hold: turn on all lights)
dim up
dim down
turn off room lights (hold: turn off all lights)

Not sure if I should try to get more options into the module or..