Jack,
Welcome to SharpTools and hope you have enjoyed it. You can create a virtual device in SmartThings/Hubitat, as the status aggregated device and use Rule Engine to update its status when other physical devices’ status changed. See the linked article below that I posted a rule example to aggregate the physical lights’ status into the virtual device.
In regarding to the the control, you can create rule to turn multi lights on or off, and add rule tile in the dashboard so you can press these tile to control all lights. You can use IF_Condition in this rule to check current lights’ status to determine if the action should be on or off based on your scenarios.
Yes, you can open and use the dashboard anywhere as long as the device has a modern browser. I have several dashboards, and some are created for mobile device specifically. On my Android phone, I opened the dashboard in Chrome, and then add that dashboard to home screen, so I can open it like opening an app from the home screen. And you can do the same thing on iOS device as well.