We've cruised twice on
DCL from Miami, once in 2016 and once this spring. Sometime in between those two cruises they redid the terminal so that security is in its own area downstairs, check in is sort of where security used to be (most importantly, it's separate from the holding pen area), and the waiting area now has more space, because check in is no longer there. It still gets chaotic right as boarding starts, but it was infinitely better this year than in 2016.
Disney does offer transportation but only between MIA and the port. FLL CAN be significantly cheaper to fly through, but that depends on what you have access to for your home airport. For us, FLL was significantly cheaper one year, and had horrible schedules the other year. FLL is a 30-40 minute drive without traffic from the port. MIA is about 10 minutes from the port. Both times we cruised we had a rental car for pre-cruise activities, and then just parked it at the port for the 4 nights. Both times we cruised we spent the night before the cruise at the Doubletree Grand Biscayne Bay in Miami, in a bayview room and could see the ship from our room on cruise day. The first time we cruised from Miami we flew in 2 days before the cruise, and spent the day before the cruise exploring the Everglades and Key Biscayne (great state park). One of the nights we had dinner at Bayside Marketplace.
This year, we flew down to Orlando Saturday night, before a Wednesday cruise out of Miami. We spent Sunday and Monday in the Disney parks and then drove to Miami Tuesday. The drive, minus the last 15 minutes, between the 2 cities was mind-numbingly boring, and took a little over 3 hours, not including stops. There are probably more cows along the FL Turnpike between Orlando and Miami than there are people in NH. As a previous poster mentioned, there are rest areas every 40-60ish miles. Several of them have an Earl of Sandwich with a limited menu. We timed our trip so that we left Disney after rush hour, but made it to Miami before rush hour. I wouldn't hesitate to do Disney pre-cruise again, even out of Miami, BUT I would not do the drive the same day as the cruise.