With many schools going back after Labor Day, I’d say the week days in the week after Labor Day would not be busy. Local kids come after school, and many locals come for the weekend, so would say those times you’d see more people.
For years, with school age kids, we vacationed at end of August due to summer camps ending 2 weeks before school started. You knew it was low crowd days as the first time I had wondered why the parks on week days, closed at 7 pm back then.
Eh, either way, we have always stayed at RPR or PBR and one time at HRH, to get the express passes. Twice I alone stayed at SF that did not have express pass, and mornings were best to get lowest lines, and used single rider where I could. That was start of October both times.
What we all recommend is that the purchase of an express pass is best to do inside the park if you think you want it. That is because they are date and park/parks specific. The unlimited kind are more expensive. No refunds. Otherwise, the purchase of an express pass is a one per ride use. Or if you buy before, it is because you know for sure you are going that day, and have only that day or a short time at the park(s). Know that the new Hagrid ride, Bourne show, and the Flyer kids ride all do not take express.
Anyway, my thoughts FWIW.