We’ve been to both 4th of July at MK and MLK day at MK.
The crowds are manageable early in the day. As nighttime comes, it gets busier than you can imagine. MLK doesn’t come close.
Avoid the hub. Fireworks are visible from most reaches of the park. We were getting off of Monsters Inc Laugh Floor with the intention of going to the bridge to Tomorrowland for the fireworks. At about 7 o’clock, the walkway out of Tomorrowland to the Hub was closed. CMs advised that we go to Storybook Circus, which we were dubious about. Turns out it was basically empty and we had an amazing view of the fireworks.
There is no projection show on the castle - so no “need” to really be on the Hub.
It also rained for about an hour, delaying the fireworks. Many people bailed. We had rain jackets and stuck it out. It was worth it. When it was over, we had about the easiest exit from MK ever. Now, that is only our experience and due to weather. If it is a perfect night, it will take a while to leave the park.
We saw people at our resort the next day who had small children and had to leave when the fireworks were delayed. They said their little one even got stepped on. They were in the crush of the Hub. I can’t stress enough to avoid the Hub.
Fireworks were absolutely amazing.