We just did a somewhat similar but condensed trip of this. We went from December 15th-20th this year and it wasn't as bad as I anticipated, but definitely more crowded then when we normally go which is in Sept (we changed to accommodate our friends last year). It did seem like crowds were increasing our last day or two there.
Like people said, as long as you RD, utilize your FP wisely (and subsequent 4th FP), pick parks with EMH and hop and/or utilize good crowd calendar knowledge, and have realistic expectations, you should be fine.
My plans have always trumped any crowd ever, and last year just reiterated by planning prowess lol. We did MK on a Saturday, a Sunday Xmas Party, and a Tues split of AK/HS (HS was wayyyyy less crowded than I had anticipated that afternoon/evening), and the entire trip we never waited more than 20 minutes ever for a ride, and we rode every single headliner and major to medium major ride, at a leisurely but dedicated pace. Only exceptions were we skipped our FP for Safari at AK (FP line was atrocious and didn't want to potentially lose out on FOP) and we decided to wait in line about 30 min to ride Expedition Everest a 2nd time (prob why we skipped Safari).
Now we don't do many shows or anything but we did watch the Jack Sparrow Pirate Training Show, the "2:00/3:00 parade", Xmas party parade, HEA, Holiday Wishes, kids did JTA, and SWGS (we passed by during JBJB but no one in our party seemed interested). So all in all, we accomplished quite a bit with the short time we were in the parks!
Now I didn't feel extra Holiday-ly magical during this time or anything, and still enjoy the Halloween festivities more, but it was beautiful to see the Xmas decorations everywhere and so glad we bit the bullet and watched the Xmas parade on Main Street.