If there are later ATL-TPA flights that day, Southwest woudl also always flat-tire rule you (free standby) if you arrive at the gate within 2 hours of your missed flight.

ATL is not for the faint of heart, as it's not just a train stop but also mutliple floors up and down escalators, and long terminal hallways in the Southwest area. But I'd likely book the same thing you did *AND* stop at the Priority Pass lounge in the Delta International terminal