Just did this for my birthday last year. It was an awesome trip.
10 nights at BVC, then 5 nights in Key West.
We rented a car for the entire time we were in Orlando, then planned to return it in Key West, where we were flying out.
Couple hiccups. We rented an Airbnb in Truman Annex. Host canceled the day we were supposed to check in while we were having breakfast at Topolino's... We ended up at the Fairfield in Marathon. We have stayed there before, and it's fine.
Then when we were leaving Key West, our plane had mechanical issues. It was delayed, then delayed. We tried to get a rental car to drive to Miami to get a flight out of there. No rental cars available. We were delayed and delayed. Maybe the 5th delay, we ran to the rental counter, got a car. Managed to change our flight to Fort Lauderdale the next morning. Long story short, I will never fly out of Key West again. It's a small airport, so there's a lack of alternative flights and rental cars if something goes wrong. I would rather drive to Miami to fly out.
Definitely thought Disney World, then Key West was the right decision.