September can also be very warm, hotter than summer. Santa Ana winds can start in Sept., cancelling any fireworks and creating very hot, dry conditions in general (not fun if you suffer from allergies or any sinus issues). The week after Labor Day this year is a puzzler -- the lower APs are allowed back into DL that week, so expect a tidal wave of locals who want to see SWGE and their beloved DL after being blocked out of that park since late May. I don't think DL is going to be empty that week, at least not from the later afternoons onward. Not all schools are back in session by Sept. -- some schools aren't back until mid-Sept. For 2020, we'll have to wait and see how things play out this year and what the AP Blockout Calendar looks like then (i.e. will the park blockouts be split again like this year?). The good thing is that the OP will have a long time to keep watching this board for trends, tips, updates, etc.
Again, crowd calendars aren't the most reliable predictor at DLR. Better to use the school and convention calendar and the AP Blockout Calendar. And always double check here.