When you think about it, summer 2022 would be the perfect opening date. It gives enough time to build the resort, it allows you to bounce back after WDW's huge year, and more people add that to their summer vacation plans.
Also, Diagon Alley, Volcano Bay, and Super Nintendo World were/will be built in a little under three years. That gives a pretty good reference for how long it'll take to build the new park.