So much great advice for you here!
I've been many times in December and January, and I agree, a hat and gloves are nice. I've had to buy a sweatshirt once (and even once when we stayed offsite in late August had to buy a couple sweatshirts but those were at Target and
Walmart). Even in the summer, once the sun goes down it's chilly! Winter 30's are downright cold.
You mentioned you don't have a park hopper. Once you've bought a 4 day ticket, adding days isn't that much (in case you want even a half day on your "rest" day), and you can also add park hopper to your ticket.
We like so much of the counter service food, especially in Epcot and DAK. In Epcot, basically any country (other than China, and I'm familiar with Asian food as my husband is Korean, but I'm not a fan of their counter service). My kids love Japan. Sunshine Seasons, I agree, has something good for everyone. DAK we like Yak & Yeti for the sesame chicken and side of fried rice (splitting that with my youngest, we are so full afterwards). Only park we find lacking decent counter service is HS, but we just make do.
Serving size - we find portions to be, as a rule, too large. My youngest, with whom I go most often, and I tend to share meals a lot. I don't want to walk another 2-3 miles right after lunch if I've just stuffed myself. Sharing a meal allows us to eat just enough, then if he wants a snack later (he has a sweet tooth), fine! I'd rather share than throw out food. I also don't drink soda much if at all, so do water (and in hot months I'd bring Crystal Light packets and add them).
I've been before Christmas, over Christmas, and the day after Christmas. The week between Christmas and New Year's Day is the busiest week of the year. The day or two after New Year's Day things start to slow down (once we were stranded for 3 extra nights at an airport hotel, after checking out of the Boardwalk Villas, after a snowstorm in the NE cancelled all flights, so I got my husband home the next day but kids and I couldn't get home for a couple more days, so we kept our rental car and drove back to the parks for a couple extra days adding days to our tickets cheaply, and the crowds after Jan 2 were nonexistent).
I'd at least go to a park a little every day. We do that, but after 25+ trips, I don't stress to do everything (I still haven't done everything and don't care).
We sleep in. Then get out. At least once a trip we go until midnight or whenever parks close, and the last hour or two you can ride so much! It's better than rope drop.