We went on a Monday in July at opening, after lunch we hopped over to another park. I would recommend taking the monorail to Epcot. We then returned to MK that evening for Wishes and Spectro. MK was crowded in the afternoon and evening, not so bad in the morning. I'm sure September will be better, but with a lot of people rescheduling their trip, who knows. You also might have some people cancelling future trips because threats of another hurricane.
If you want to get a jumpstart on the crowds at MK, make a PS at Crystal Palace at 8:05 or 8:15. We did that and then walked on to Splash Mtn. right when the park opened. Plus it's a fun breakfast with Pooh, etc., and the food is VERY good.