The past 2 yrs we've gone the last few days of Sept. into the first few days of Oct. In comparison to May and early December we find it to be the least busy time of the times mentioned.
But, no crowds at WDW, no, you will always find some kind of crowds there these days at sometime during your stay. Lesser crowds at that time (especially mid-Sept) but, F&W will be crowded especially on weekends, so if you don't want crowds don't go to F&W.
Probably the least crowded time will be just after Labor Day. But, do expect classes from local schools at Epcot during this time.