I agree that the marathon isn't going anyway - if you're more excited/ready to do it in 2021, then that's the best time for you to do it.
I also agree with other posters that 11 weeks is plenty of time to train for a half, and 10 months is plenty of time to get ready for the marathon. HOWEVER, running is such a mental sport...I totally get not being confident in working up to the marathon, but I think you'll get there as you train for/complete more races. Not totally sure how long you've been running but for me, it was about 3 years from the start of my running regularly to running the marathon at WDW. I felt like I had to *know* I could do a 5K before a 10K, a 10K before a 10-miler, etc. It was a very slow build for me.
Lastly, I agree that it's smart to do a half marathon before the marathon for a few reasons - you mentioned trying out gear/clothing/fueling. That's a big part of it, you don't want to be, like, trying new shoes for the first time during the marathon or something. You'll also want to submit a proof of time to Disney for your marathon corral placement, and you need a race of at least 10 miles for that. I think finishing a half will go a long way in your being confident that you can finish the marathon...you'll know a little better what it "feels like" to run long. And finally, just sort of getting into the idea of what marathon training will sort of look like will be a big thing (obviously you'll have to devote more time to the marathon training, more miles, hours, etc. compared to half marathon training, but it'll at least give you an idea) - the training was always the hardest part for me, the race itself is just fun (assuming training goes well!).
The plan I'm currently leaning toward:
--Switch to the 10K in June
--Register for the local half marathon in August
--Build a 20 week training plan for the half that incorporates the 10K
--Plan a non-WDW trip for late 2019/early 2020 to satisfy my need for travel
--Train for and run a variety of local races from fall 2019 through fall 2020, including at least one more half
--Run the WDW marathon in 2021
Sounds like a great plan!!