Consider Pisa (PSA) as a viable alternative to FLR. People Mover from PSA to the central train station and then a direct train with a few stops to the center of Florence.

Would you consider LAX and the Alitalia LAX-FCO non-stop that operates 4x-7x weekly? That can be a good use of Delta miles as a U.S.-originating round trip - no international origin surcharges. Florence can be a 90-minute train ride from Rome's central station.