I took my son in ATL last Monday morning for a flight back to college and I was amazed at the number of homeless we passed on the way to check in. When we took the train from the hotel (that runs between the airport and rental car facility) there were several folks asleep on the floor as we were waiting in the train.
That is not typical of airports.
Avoid MARTA if at all possible.
Did you stay at the Springhill Suites?
Every time I’ve flown out of Atlanta, I would drive up the day before and stay at the Springhill. Their parking rates are cheaper than over at the airport.