Easiest times of year are Feb 1 to the Thursday before Presidents' Day and about July 8 to the end of July. Of the times you mention, January after marathon weekend, and all of Sep before about the last five days, are two moderate DVC demand times when much is available at 7 months out. Earlier in Jan and late in Sep are during DVC's high demand quarter of the year which runs from late Sep through marathon weekend in Jan. "A week in early December" would be seeking a room during THE highest DVC demand week of the year and little to nothing may be left at 7 months out -- even SSR has some risk of having a day or two filled before 7 months out.
Even as to that January and September moderate time you can have problems at 7 months out for certain usual suspects which can be gone by 7 months out any time of the year: AKV value and club level rooms, BWV standard and boardwalk view studios (and resulting 2BRs), VGF studios, and BLT standard view. Also possible issues at times for BCV studios and 2BRs and BLT theme Park view.