I was there at that time in 2018-9. What we found was the parks were packed beyond belief, with mornings being lighter than nights. We actually hit a phased closing on New Years Eve, but for the Magic Kingdom only. We were in Epcot that day and didn't get a pop-up from the app until after we had heard people talking about it. If there were other closings, we were unaffected.
On your first day, concentrate on only FastPasses and events like parades and fireworks. Walk, shop, enjoy. It will help you retain your sanity for the week. On the last day, pick a special place for a meal. It doesn't have to be the best place, but a place for photos and memories. Again, sanity saving. You are taking a moment to say goodbye.
My suggestion on the FP's is get early afternoon passes, then try to roll a new one all evening long. Forget about getting a E ticket FastPasses once you use up your first batch. When the park opens, you are good for one ride, maybe 2. A little breakfast, site seeing and a second or third ride gets you to lunchtime. Then your FP+ kicks in and you are good until dinner time. Then start rolling those passes while you eat. They won't be great, but they are better than nothing.
Remember, there are problems and mistakes to be had and made this week. We had a FastPass for Spaceship Earth and something went very wrong. The FastPass line was longer than standing in the regular line. It went all the way around Spaceship Earth. There was a certain amount of Christmas Time Hamminess going on, where people were getting in the line on purpose to get a picture or a laugh, rather than to really wait or figure it out. They offered us 2 FastPasses to make up for the obvious glitch (via the app), but the only rides available were The Seas with Nemo, Turtle Talk and one or two others. The take away on this is, there is zero capacity to fix a FastPass issue during this week.
Another thing that stands out to me is the parades and such. Go get in a line when these start. The lines dip a bit that these times, but more importantly, you aren't trying to navigate an area packed to a capacity AND a parade crowd. If you are watching youtube videos about Christmas Time Crowds, they all seem to filmed in the mad minutes after a parade or other event to make it look more crowded. Don't watch the youtube videos. It isn't that packed, it's "differently packed".
One thing I will tell you is, if you are interested in Epcot's Candle Light Processional, it is rather hit or miss. Not the performance but the actual crowd. We had a dinner package with tickets for it, but the seating area wasn't full (about 2/3 to 3/4 for the whole show). At first I got mad that I paid extra for a thing that wasn't "a thing", but then Christmas mode kicked in and I was cool with it. What you should take away from this is, if you don't have the time/money to get the package, sometimes, perhaps very rarely, you can walk up at showtime and get a seat. It isn't a great plan but if you hit Epcot a couple of times on your trip, you might be able to skip the dinner package AND the standby line. It is very opportunistic thing, so I wouldn't count on it, but keep the show times handy and check. It is very far from guaranteed but if you can pull this off, you will be a legend for the week.
