January has always been a slow month. Right after Christmas break/week, it shouldn't be bad at all. I don't know about the Christmas decor. I don't know when they take that down.
Marathon events take place January 8-12, so availability is already tight. There is not much holiday decorations around. Early on, some resorts may have the tree up, but overnight it will be taken down.
By Jan 6 the hotel decorations will be down. You might find some in the parks that first day or even two but those will be in the process of being taken down during those days.
January has always been a slow month. Right after Christmas break/week, it shouldn't be bad at all. I don't know about the Christmas decor. I don't know when they take that down.
The parks will be crazy crowded until all the holiday visitors finally go home. I would expect the 1st part of your trip to still be busy, but slow down during the 2nd week of January.
We visited the 1st week of January the last 2 years. The holiday decorations were still up both years, but were in the process of being taken down. There seemed to be less decorations with each passing day!