Disney World 12/21

Unfortunately if you dont allready have reservations, fort wilderness may be booked solid. Especially on the dates you will be there. Its busy around the holidays and spots fill up fast. Likewise, with the ticketing system now, you need to book the park in advance. I have no clue if the parks will reach capacity, but around Christmas time they may. Especially now with the reduced capacity.

As mentioned, Disney's Fort Wilderness is a beautiful campground. We have stayed there a handful of times. It's quite expensive, though.

Here's an older write-up about it:
https://learntorv.com/camping-at-disney-world-fort-wilderness/

As for driving down to the Keys, we like to go down to Miami first for a night or two. We have always stayed at Larry & Penny county park. It's nice and has decent spots. It's right behind the Zoo and you'll occasionally hear the lions roar. We do that so we're fresh and awake for the drive down through the keys and are able to arrive at our campground there earlier in the day.

In the Keys, we've only stayed at Bluewater Key resort. It's absolute luxury (with a price to go with it!). Be aware that December is considered peak season in the Keys and pricing reflects it.

If doing Disney stay at Fort Wilderness when going to the park. Its worth it just to see the campground, if staying in the area for a while, I'd stay at the Fort the days you are going to do the park, and then elsewhere the rest of the time.
Central FL has lots of incredible natural springs that the manatees move into in the winter, it is worth the time to visit some of those if you can work it into the trip.
If you have the time the Keys are incredible, but you need to be booking sites now.

If your traveling from CA to Disney World, you should stay at Fort Wilderness and experience Disney to the fullest !

If you are looking to NOT spend the money and stay outside Disney. make sure you factor in the $25.00 daily to park on Disney property

From there it's a short, 6 or 7 hour jump to The lower Keys..