"I am trying to avoid ... exorbitant "convenience fees" for the tolls."
Have you considered purchasing your own personal SunPass to use in your rental car? If you can find a reasonably-priced one-way rental from MIA to FLL/PBI, that's probably what I would do -- especially if I had the amount of luggage that you will have.
Link to SunPass retailers:
https://www.sunpass.com/sunPassRetailers
(Of course, if you rarely drive in Florida, the initial $19.99 cost of a SunPass transponder might make this an unworkable solution for you.)