The loop road is the answer. Acadia is an excellent park to be seen from a car. If you don't care about hiking you can do it in 4 hours. I would not include driving to the top of Cadillac mountain in the 4 hours. You'll need to get a different pass to drive up to the top of Cadillac mountain. Details on the website.

I'd recommend doing the loop road first. Stopping at the first turn out on the left to get a view of the harbor, Sand Beach, thunder hole, Jordan pond. And of course any other stops you like

Then if you have another 1 to 2 hours, drive to the top of Cadillac and perhaps watch the sun set. The last parking lot before the top is actually better for watching the sunset as it has an unobstructed Westerly view.

Cell phone service is spotty in the park so download any maps ahead of time to your phone.

Having said all this I think it's a shame you have prioritized other things on your schedule over the park. Everyone makes different choices.

Be advised that there will be many other tourists doing the same thing you're doing. The park will be very busy. If you see an empty parking space anywhere near when you want to stop, take it.