I ran Chicago in 2017. I think it was a humid 59 or 60 at the start but by the end it was 75. My goal was sub 2:30. Blew up, which I attribute to the heat, and ran 2:34. I trained 100 MPW in humid DC area over the summer too. I loved the Chicago marathon event itself. The city, the organization, the course, etc. However, it's just a really risky race to run if your primary concern is the time you run.
Early October has now basically become an extension of summer. If the temps are not high, the dew point and overnight lows now tend to be. I figure the only safe bet to avoid heat is a November race. Winter months are great but can also come with greater chances for wind and rain. November seems to be the best option to race a fast marathon. If only Chicago were in November, I'd consider doing it again.