MIA from a road approach point of view is a horse-shoe arrangement, so which departure terminal is not too important for timing. Hire-car return is relatively close, and a short bus-shuttle ride.

I wouldn't want to drop the car later than 14.00. And that's travelling solo. I suppose from the Flamingoland reference that you aren't travelling solo, so I'd add more time.

Right now, I'd want to add more time still for potential TSA short-staffing. I don't know when you are travelling, so that may not be relevant.