That's what we did last year! My kids were 15 and 12 and we had a great time! We went in with reasonable expectations. Like you guys, my kids had been to Disney many times so we didn't feel in a rush to fit everything in. We spent a lot of time enjoying the resort decorations, and eating at new restaurants we've always wanted to try. We were lucky enough to have great pool weather so that helped when we just needed a break from the crowds.
We also did MK on the 30th. I'm not gonna lie...it was mobbed. And it got more crowded as the day went on. Plus it was misty raining all day. But we still had a great day. We made FP+ for the noon-time parade so that we didn't have to wait on the curb for hours to see it. We also planned 2 sit down TS meals to try to break up the long day. We got to MK early and we were afraid to leave (in case they closed the park due to crowds). The crowd for the fireworks was INSANE, but it was the best fireworks show I've ever seen. ever. No regrets about being there at all.
Plus, this is the last year for Osborne Lights. Last year was our very first time seeing the show and it was awesome!!
On New Year's Eve we didn't even go into a park. Instead, we made an early dinner reservation at Artist Point and just spent some quiet time with the kids. We left the next morning, so it was a nice relaxing way to spend our last night. Since we had seen the fireworks the night before (and stayed up really late) we all ended up conking out earlier than we had planned.
I am so glad we made the trip. My DD is in high school, so I totally get only being able to travel during school holidays. We had a blast! Being in WDW during Christmas week is crowded, but it is so filled with happiness and holiday spirit that it's worth it....at least once!