My first trip we flew out my hometown on Dec 25th at 11:30PM, arrived in the morning of the 26th and went back home on January 5th. It was busy. Like, parks hitting capacity before noon kind of busy. Lines for everything. But it was also magical. I remember the weather being great. Some days I wore a light sweater, most days I wore capris or pants, but overall it was very comfortable. Rain a couple afternoons, but the storms just helped clear out the park.Like others have said, the decorations are beautiful and there's tons of atmosphere to take in. I think if you've been often in the spring you've probably done lots of the e-ticket attractions. This trip could be a chance to try new things you don't usually make time for and holiday-specific events that you miss if you aren't there during the holidays. You can still do the e-ticket stuff, just make sure to prioritize them in the early and late hours. If you're there for New Years, make sure to rope drop the morning of Jan 1st--we have the greatest picture of the two of us alone in a Jurassic Park vehicle, and we basically had the park to ourselves until around noon.
General tips: Take advantage of FP+, and book resort and dining reservations as soon as possible. Also remember that everything is priced to match the crowd level--it isn't just the busiest time of year but the most expensive. Your resort and meals will without question cost more than you're used to. It'll be different than you're used to, but different =/= bad.