Unknown is whether there will still be a covid impact for Dec 2021 which could lower the crowds at WDW, but in normal times, Dec 23 to Jan 2, is a time for huge wall to wall crowds at WDW, larger than crowds at any other time of year. Think of yourself standing at the base of Main Street in MK at 11 a.m. and looking down Main Street to the castle in the background. You will have difficulty seeing the street or the sidewalks; instead you will see a mass of bobbing heads all next to each other occupying all the space, all the way to the castle. See example photo attached.
Moreover, arriving Dec 19 and going to the 26th is also very crowded but means it will just get worse and worse each day you are there until Dec 23 or 24 and then it becomes impossible. If MK returns to normal crowd patterns but fastpass is still gone, then don't expect to get on anything (even the lesser attrractions, e.g., Swiss Family will be over an hour) with less than an hour wait (Soarin and Test Track can get to 3 1/2 hour waits) except possibly when trying early in the morning right when the parks open to on-site guests.
The other problem will be getting a room reservation unless making one at 11-months out. At 7-months out, you will likely be limited to SSR, possibly OKW and possibly Kidani savanna view, but even those have a risk of disappearing before 7-months out.
I cannot speak to the crowds in Hawaii or Aulani, but during the applicable times mentioned, it can be difficult to get anything at 7 months out except possibly Ocean View and for even that view studios can be gone, although chances for some other rooms can be better if start date is Dec 19.
