Tour de Moore century is in September. It is in Southern Pines, NC. That is near Pinehurst...where the U.S. Open is next week.
The terrain is beautiful....with a scenic rolling course through the NC sandhills. The ride is well supported. don't think it meets your criteria for 500 riders or more. You would certainly enjoy your stay.
Do a google search for sandhills cycling
I'll be there doing it I'm sure.