the beginnning of the month to mid sept is not crowded based on past years.
the very end of sept will be crowded especially for the onsite hotels.
if you want to go then and stay there, book soon.
the
HHN begins on sept 26-27 only friday and saturday as it is the opening for the hard ticket event.
if you want to go at the end of the month to stay onsite, go during the weekdays and you will avoid the crowds at the hotel.
if you want to stay off site, again, go to the parks during the week to avoid the crowds.
i have found that tues, wed, thur to be lighter than other days in the park near the end of sept.
if you go the 3rd week, and stick to weekdays, no problem on crowds.
this is how it was for me on past visits. also, many posters have commented about the same as what i have experienced.