Tuscany itinerary

Except for Montepulciano and Orvieto, your towns are nicely bunched. Save Montepulciano for another trip.

Considering your large group that includes a toddler, think about visiting Orvieto from Rome on one of your days there (not ideal but perfectly doable), taking the train from Rome to Florence, staying in Florence (+ an extra night or two) and visiting Pisa and Lucca by train, renting cars in Florence and go stay in Volterra and visiting other towns from there. Return the cars in Florence and take the train to Rome. Saves money, fewer changes of accommodations, less time spent driving, no time spent with your cars parked somewhere full of your luggage, more flexibility in case someone gets sick or cranky. Also, trains are more fun for toddlers than cars.

Just wanted to add that one of the older folks has a bit of limited mobility. I think that's an argument for cars instead of trains for Tuscany, since we might be able to drop them closer to the cities—even if we have to park a little farther after dropping them.

I would look at isolating the car rental to only rural Tuscany and find a way to bunch those days and pick-up and drop off in the same place and use the train for the rest of the trip.

I would suggest either train from Rome to Florence and then pick up the rental car, spend time in rural Tuscany, return the car in Florence and train back to Rome. This puts cities first and towns last.
Or train Rome to Chiusi, rent a car for rural Tuscany and return it and train onto Florence and then back to Rome which bookends rural towns with cities. (You can also do this with Orvieto but Chiusi-Chianciano Terme provides a bit more choice in rental agencies and is closer to Tuscany since Orvieto is in Umbria and offers only Hertz.)

I sympathize with the idea of using the car to get closer to locations but if you read up on ZTLs I think you'll find you will not be able to drop your party off at your hotel in Florence, or city center in small town Tuscany, and then drive back to the parking lot without incurring heavy ZTL zone violations. Trains and then taxis as necessary are probably the way to go in Florence if you are concerned about mobility because you will/should not ever consider driving into Florence proper but instead going just to exterior parking lots following only the predetermined safe path.

Also keep in mind that 3 nights is only 2 full days without packing and changing hotels. If you have 9 nights total I would think about whether 3 bases with 2 full days at each makes the most sense or whether a 4 and 5 set-up makes sense if day tripping is the goal. You could stay extra time in Florence and visit Siena (bus or train), Lucca and/or Pisa as day trips and the rent the car for some Tuscany daytrips from one well chosen location.

Usual driving tips apply - make sure all drivers have an IDP before leaving the states, read up on ZTLs in general and then in each town just before you visit, and add at least 25% travel time to Google maps driving times for rural travel and leave time for getting lost as well as feeling lost even when you're not.

My $.02, all these destinations are great so it's hard to go wrong but try to limit your logistical hassles,
=Tod

Interesting ideas. Thank you.

Did you already see this very useful recent post about ZTL maps? https://community.ricksteves.com/travel-forum/italy/ztl-maps-for-small-puglia-towns

Try plotting out the various ways of doing this trip, with drop-off points, walking distances, driving times, hotel locations, rental car locations, and places where a taxi would be helpful. I've now spent a month in Italy four times with varying degrees of foot or hip injuries, so I can picture what that would mean in the cities and towns you want to visit. Also did one shorter trip like yours with 6 people including a baby and a preschooler.