As is said above, the FL Keys are not a "resort" experience. That being said, I travel from CO to FL almost annually and always enjoy myself. This is my perspective as a diver who travels to FL to dive. People who live in the area may have a different experience and pov, just as those of us who do day trips to Winter Park, Copper, or Vail do compared to someone who travels from out of state to get here.
There is a nice variety of diving in Key Largo from shallow reefs to deeper wrecks. The biggest issue in FL is the weather, which is inconsistent compared to Cozumel. Bonine is a good thing to pack if anything higher than 2-foot seas causes you sea sickness. Since the boats in FL tend to be bigger than the smaller fast Cozumel boats, operators will go out in 5-6 foot seas. I have experienced the best weather and sea conditions from June-August in FL.
I am glad to hear Amy Amoray's was a good experience for another diver. It has been a long time since I stayed there, so I would not be able to give an accurate review.
When I go to FL, I will often book things separately, although plenty of ops have a partnership with a hotel and those are worth looking into. I like getting a VRBO (or similar) because you do end up with a decent amount of down time and it is nice to have a place to spread out.
You are going to need a car because you need to drive to most places and while pedestrian access has improved over the last decade, driving still gives you the best access to the area. Not everything is within walking distance. Also, you need to get to Key Largo from Miami anyway. One thing to consider when renting a car is the fee the rental company charges (daily) for the automatic road tolls you will encounter between Miami and the Keys. You must pay the fee each day, even when you do not use a toll road.
I highly recommend Horizon divers. They have boats that go to reefs and to wrecks. They have guides you can hire and I found them to be a very well run and friendly op. They cater to a variety of divers from vacation divers to people making a day trip who live within driving distance.
Florida diving is definitely different than going to the Caribbean for vacation, but it is enjoyable and worth checking out.