We’ve been in WDW between Dec 30 and Jan 3.
Starting on Jan 1 the crowds drop dramatically from their peak with each day being better than the last.
Crowds start to ramp up for MLK’s birthday weekend, but not too bad. As noted, crowds do also increase for the marathon, but again, not too bad.
All parks can easily host 20,000 guests comfortably. That is only 25-33% of capacity depending on the park.