It’s gonna be packed. We go every year for Christmas and NYE.
12/31 is unlike anything you’ve ever experienced. Disney is always crowded now, but NYE is a whole different animal. People show up at rope drop and it stays crowded the entire time. Some will come and find a table or spot to stand for the fireworks from the morning on.
BUT… BUT… once 6 or 7pm comes, it turns from an incredibly crowded park into one gigantic party—especially at Epcot.
My recommendation would be to go to MK on 12/30. They do the exact same show… then go to Epcot on 12/31.
Go early, take breaks in the afternoon, then go back late at night.
FP+s and EMHs are your friend.
Park Hopper is worth it.