Thanks, the reason I asked about your departure is that you have to be in the check-in line at 7AM on your flight home day, then. That makes the decision about ditching the car and sleeping the last night, for you. You have to sleep at the airport, or a near-airport hotel - but make sure their shuttle runs at 6:30AM on that day of the week.
We were not happy about having to miss an evening in a town or city, but we did spend our night after Tuscany at the FCO Hilton. We returned the car sometime after 6PM (I forget exactly), making a few stops on the drive from Sinalunga. Type "FCO Hilton" into the Search box top left for many comments about that and other hotels near FCO.
EDIT: Sorry, when I wrote "have to sleep at the airport", I was assuming that you were thinking of sleeping in Tuscany up until your departure. Staying a night downtown in Rome makes better use of your time. It's a big city where it's not that hard to get a dawn-patrol taxi reservation, or even the Airport Express train early in the moning.
It does make the choice of a car return location different. I wouldn't want to drive in downtown Rome under any circumstances!