We've been both weeks. The earlier week it rained more, but the parks were open later. The later week it rained less, but the parks seemed to close just a little earlier. Also the soft opening the day before food and wine opens was great.
September is my favorite month. The heat is no different than Aug in most of the US states.
Go at rope drop
Leave at 1pm for pool/resort time
Get an ADR at a restaurant near or in your park for a 5pm dinner.
Back to rides for the night.
Honestly, if I was going in Sept, I would pick 3 rides for evening FP because in the AM, you can knock out so many rides. Then at night you can have a more leisurely stroll to your FP rides, parades, fireworks etc... I usually start the parks in shorts/t-shirt/sneakers, shower after pool and go back in capris and flip flops. It feels like you are getting 2 park days in one.
The only time we don't leave the park and go back is AK day. We go until about 3pm, go to the pool and then to DTD for dinner at Raglan Road and shopping.
Maybe you are right. I guess I assumed it would be crowded early but I guess most people stay local since schools are just back in session. And it is before MNSSHP and F&W too.