Rental car. We have flown into MIA for cruises on the Magic multiple times, following with a WDW stay. After the cruise, we head to the car rental center at the airport to pick up the rental car for the drive to our resort. We usually do a one-day rental, dropping off at the Dolphin since it’s near BWV where we stay and we use Alamo. The drive is a leisurely 4 to 4.5 hours for us, although I don’t keep precise track because we do both lunch and grocery stops before we get to the resort.
One-day rentals have usually been in the $45 to $75 range, plus gas and tolls.
Alamo/National has a free shuttle between the car rental center at MIA and the Port of Miami. I think other can rental locations offer shuttles as well.