...I know it will be different for you because you are flying into Miami. You could drive to WDW and then fly home out of MCO. Maybe others will have better ideas.
I've done all sorts of cruises with either WDW at the beginning, end or at both ends of my cruise. No matter how I visit WDW I usually use Disney transportation to get from terminal to WDW or vice versa and then magical express to the airport.
This is my recommendation. It will be far less stressful and time consuming than driving BACK to Miami once you're ready to go home. In general these open leg flights where you fly into one airport and out of another are just about the same or slightly more than flying into and out of the same airport. You may have to pay a one-way rental return fee, but again you'll be saving yourself several hours of travel time.
I'll be doing something similar next year. DW will be attending a conference in Orlando starting the day we disembark, but I plan to spend a few days enjoying WDW so will need a car to get there each day. Consequently, I'm going to get a rental car at the port and return it to MCO. There is a one-way fee, but I actually save money and time compared to taking the DCL shuttle to MCO, then renting a car there just so I can return it back to the same office without a fee.
Do you mind me asking how you knew there was a one-way fee? We are renting from National at PC and returning to MCO. Our rate was $39 and now I'm worried about a hidden fee??
Each rental company is different. Some charge a one-way fee and some don't. There is such a large demand for one-way rentals between MCO and PC that some companies don't charge a fee.
If there is a fee, you will see it during the reservation process when they show you the daily rental rate, taxes and fees, optional items such as GPS and child seats, and a one-way fee, if there is one. It may or may not show up in the initial quote, but you should see it before you complete the reservation.
I've had good experiences with Enterprise, National, Alamo, etc. I won't use Budget anymore because they did have some hidden fees, and the clerk had a smirky attitude when I objected to them.