I have done the rental car thing a few times. It actually works out pretty good. If I remember right, I used Enterprise the last time I did this. They will actually pick you up at the Pelican and take you to their office to rent the car. Then I returned it at the airport in Tampa and took a shuttle to Fort Desoto to pickup my car. Then I drove down to Key Largo with my trailer to pickup the boat.
If you have a kayak and you plan ahead with some nice rubber-block holders and some long straps, you can put the kayak on top of the rental car. Drop off your boat at Fort Desoto and then return the car to the airport. Take a shuttle (or Uber these days) back to Fort Desoto.
I know this sounds like a lot of hassle, but it really wasn't that bad. I have done this three or four times.
One year I took the bus to the Miami airport. Flew to Tampa and took a shuttle to Fort Desoto. The bus stopped right near the Pelican and the cost was roughly the same as renting a car.
But of course it is much easier if you can hitch a ride with other WaterTribers. I have done this a few times as well. I always insist on paying for the gas.
Of course the absolute best option is to have your family down at Key Largo. They can have a great vacation while you are busting your butt.