These are your options on Jun 24, 2023 from FLR to BOS with one stop only (I excluded the early morning departures):
TAP Airlines: dep. 11:45AM arr. 8:20pm (via LISBON)
Delta or KLM (codeshare): dep 11:20 arr. 4:55pm (via AMSTERDAM) - (FLR-AMS operated by KLM)
AirFrance or Delta (codeshare): dep 9:50 arr 3:00pm (via PARIS CDG)
AirFrance or Delta (codeshare): dep 12:10 arr 8:30pm (via PARIS CDG)
Swiss: dep 10:35a arr 3:20pm (via ZURICH)
Swiss: dep 1.55pm arr 8:05pm (via ZURICH)
Swiss: dep 2:50pm arr 8:05pm (via ZURICH)
Lufthansa: dep 10:30a arr 3:25pm (via FRANKFURT) - (FLR-FRA operated by AirDolomiti)
Lufthansa: dep 1:30p arr 6:00pm (via MUNICH) - (FLR-MUC operated by AirDolomiti)
Lufthansa: dep 2:40p arr 8:25pm (via FRANKFURT) - (FLR-FRA operated by AirDolomiti)
I rent cars through this broker: www.autoeurope.com
You need to check in about 75 min prior to departure. Allow about 30 min for the car return procedures and the 5 min shuttle ride.
The drive from Castellina to the airport is only about 50 min without traffic. However there is traffic getting into Florence between 7:00am and 9:00am, therefore give yourself an extra 20 min plus another 20 min to get lost and find your way to the rental car center on Via Palagio degli Spini. It's not hard at all if you choose to go to the Firenze Nord exit and come down to the airport from there. So my suggestion is to leave Castellina 3+ hours prior the flight departure.
EDIT: Since you fly on June 24, that is a Holiday in Florence (St. John's the Baptist, Florence's patron saint day), therefore there should be no substantial traffic. The holiday is only in the municipality of Florence, not surrounding cities. But it's a Saturday (which is not really a holiday in Italy), therefore traffic shouldn't be too bad.