Imo that flight would be fine. Ideal for me, actually, because I don't like waiting for hours and hours at the airport. It's easy to get back to the Miami airport around 9am, because it's only a 20 - 30 minute drive to the airport from the port.
Express walk-off just means that you are carrying off your own bags and therefore will not need to pick them up downstairs at the port before going through customs. You can exit the ship early or late if you keep your bags with you. You can do the
DCL transfer bus whether or not you have DCL take your bags downstairs for you. They will NOT send your bags to the airport for you anymore. They used to have that service available, but not since reopening. You CAN have DCL check you in for your flight, with most airlines.
In your shoes, I would not put my bags out the night before (waiting for your luggage group to be ready downstairs can slow down disembarkation), and I wouldn't have late seating breakfast (if that's your assignment). I'd have breakfast at Cabanas (they open at 6:30) or early seating at the MDR if that's your assignment, and then line up to disembark asap. That will get you to the airport with plenty of time.