My original plan was after a week in Rome to take the train from Rome to Orvieto, rent a car and start my tour of the Hill Towns from there. However, many negative posts about the Hertz office in Orvieto has made me reconsider. I'd like to avoid as much city and autostrada driving as possible. Any opinions re: Civitavecchia vs FCO as a starting point?
If you're heading north to the Tuscan hills, why don't you just go ahead and take the fast train to Florence? Pick up your rental car at one of the city center locations when you're ready to leave town. The rental agent will give you directions on how to get out of town avoiding the ZTL.
We turned our rental car into Orvieto Hertz and had no problems at all. We had picked the car up at Hertz' Sansovino location which is about a 2 mile taxi ride from the Florence train station.
We stayed between Certaldo and San Gimignano which was about 30 minutes south of Florence. It's a great central location and virtually every farm in the area had apartments and rooms for a secondary source of income.
Rent at or near the Tiburtina station. It's next to the freeway entrance.
I should have been more specific about our itinerary: Rome - Orvieto - Pienza - Assisi - Rome.
In your case I would rent in Rome and return it in Rome.
I generally use a consolidator:
www.autoeurope.com
www.kemwel.com
Both are now part of the same corporate group but I check prices in both as they vary, then choose the cheapest option. They work the same way. They contract with the major companies (Avis-Budget, Europcar, Hertz, Sixt, etc.) but they have better pricing than directly. Lately I've been choosing the Europcar selection as it is often the cheapest when you select the full zero deductible insurance option, which I select for peace of mind even though slightly more expensive (otherwise the deductible is often a stiff 1000 euro).
To see where rental offices are located within Rome you can use the sites below. Just zoom into the map of Rome and choose the one you find most convenient to get out of town. To go to Orvieto you will be taking the A1 freeway in the north direction.
I put down only Europcar and Hertz, since they are the two with the best pricing:
https://www.europcar.com/station-finder
https://www.hertz.com/rentacar/location
When you leave Rome at the end of your week visit and go to pick up your car, take a taxi from your hotel to the rental office of your choice. Take all your luggage (and spouse/partner if any) with you, because if you stay downtown it will be difficult to drive back to the hotel to pick up your luggage. Most of the historical center is closed to traffic.