they are currently selling Q inventory DCA-ATL-CHS at 5,6,7 PM as well as the 8PM where OP evidently has a reservation ... I don't have EF, and I know seat maps aren't any kind of authoritative in terms of availability, but all three earlier jets appear to have 10+ unassigned coach seats
there are also award seats available on those three flights tomorrow, so as several others have stated, an SDC request at the 3-hour window for any of those flights seems like a high-probability option