This itinerary should be fine if there are no flight delays. But I'd keep checking to see if availability opens for a one-stop routeing.

At ATH you will be going landside then through security. No security needed at GVA, not sure if they do extra checks at the gate for US flights.

Main issue would be having to go through passport control twice since ATH-GVA is intra-Schengen. August can be busy with infrequent flyers. While you'd be first off the plane coming from C class, this doesn't help if you get stuck behind pax from other flights at immigration.