So many variable to consider.
We typically go one of the first 2 weeks in December due to crowd volume. Never any problems and we get to catch all the decorations. We have been twice December 26th - January 2nd. The day after Christmas, New Year’s Eve and New Year’s day were an absolute zoo with a capital Z. Never again during that time. I think back to all the money we spent to go and were only able to get on a few rides - for us it was depressing. The parks closed to new visitors during the day for several hours due to the capacity issues.
This coming January, two in our group are participating in the 1/2 marathon and full marathon. All indications show that the first day or so will be crowded (1/6, 1/7 and 1/8), but then everyone leaves. We are there for another full week, so we should be okay. I hope the indications are correct.
I think it is best to experience the various times at your own discretion. What you may consider crowded, may not bother anyone else.
Good luck with your decision.