Stressful to drive a rental car in Tuscany

This depends more on you and your personalities/comfort level driving.
The countryside driving itself is not stressful, but coming and going from a city can be. I won’t pretend my husband and I have never fussed while navigating or parking, but we find it worth any hassle to see places we would not otherwise see.
Perhaps you can tell us more details about your itinerary so far and we can make suggestions—some will even have drivers they have used.

As Valadelphia said, driving around the countryside is easy. The difficulty lies with driving in any urban area, be it city or village. Inadvertently entering ZTLs can really wreck your budget; but you wont find out until months after you return home and get letters in the mail.

How many days will you be in Tuscany? How many days did you plan on driving outside Florence or Siena? Most importantly, which towns and villages will you be visiting? How many have train or bus service?

You might want to read up on the difference between driving in the US and in Italy https://throneandvine.com/driving-in-italy-made-easy/. Also, study the road signs https://en.wikipedia.org/wiki/Road_signs_in_Italy.

That said, my husband and I always say it take two of us to drive in Italy — and we lived there for awhile — because unless you know the routes, the driver needs a full-time navigator. There’s just a lot of signs to watch for and interpret. We liked to have the navigator use Google Maps rather than a satnav in the car speaking Italian.

Both Florence and Siena are better places to stay and see without a car. But we successfully stayed in Siena having found a lodging with a parking spot we could rent that was only a 10 minutes walk from the B&B. I would suggest use one of the two cities as a base with you car and simply stay in the other and enjoy it.

Another option is Tours By Roberto https://www.toursbyroberto.com/. Sign up for one of his joinable tours. They are small groups, reasonably priced, and will give you a fine day out in the Tuscan countryside.

In case you decide not to rent a car, I recommend that you check out the bus system in the area. It is extensive. The main bus terminal in Siena is at the Piazza Antonio Gramsci. The ticket office is underground, accessible through a small kiosk.

Driving in the countryside is not stressful. You just need to be aware that many road lanes tend to be a bit narrower than in America, and there are no shoulders on the sides, moreover Tuscany is very mountainous and hilly, so there are lots of curves, including hairpin curves, so you can't be too distracted while you drive or you might end up in a ditch or hit a side wall. Just drive slowly.

Rent a car on the way out of Florence (don[t have a car while in Florence) and I wouldn't worry about ZTL (restricted traffic zones) in the historical centers. Just learn the signs and park outside the ancient walls of every town.

If you plan to have a car, maybe it's not a great idea to stay in the city center of Siena, since the city center is forbidden to cars. If you plan to rent a car, find accommodations somewhere in the country side hotel, or a smaller town or at least outside the city center of Siena.

I won't rent a car in Italy. When I was younger and lived in Germany 35 years ago, I drove in Italy, but not now.
1) parking is a huge problem
2) navigating narrow roadways and roads in towns and cities that are limited to others (not you).

3) You are likely to get a ticket up to a year after the accused violation for hundreds of dollars.
4) I drove in the UK and tried to keep the speed limit, but got a ticket for going 35MPH in a 30MPH zone for $58. I have heard of some getting tickets for going 3KMPH over the speed limit in Italy.

Take the train or bus. Another option is to stay in Florence or Siena and take day tours to its many towns and cities.
https://www.tripadvisor.com/Attractions-g187902-Activities-c63-Siena_Tuscany.html

https://www.tuscanescapes.com

Roads are fine. Other drivers are the problem. If you’re comfortable staying calm when someone passes you on a curve meant for a goat, rent the car. If not, let someone else drive.

Why is it such a problem to drive a car? If this fear came from someone living in North Korea, where nearly nobody has ever driven a car I could understand it, but from Americans? People who are inside a car virtually most of their lives and use the car to drive any distance above 500 feet? That I don't understand. Just relax. It's not that difficult. When you return home you will ask yourself why you even questioned your ability to drive in Italy. They drive on the right side of the road there too.

We have rented a car several times for Tuscany- also seniors. It is not stressful and we have never received a ticket.
However if I was staying in only Florence and Siena I wouldn't want a car.
You can't drive in either and getting in and out, parking would be a pain.
A car is useful for smaller hilltowns/hilltown stay.

If you only want a day tour or 2 book a driver/tour- plenty of those from both Florence and Siena

Check toursbyroberto.com- great reviews, he books up fast

If you are comfortable driving at home, you can drive in the Italian countryside.

Thank you everyone for taking the time to give me advice & opinions!

A suggestion I would have is look at the google route before leaving. We have been rerouted via interesting non direct routes from point a to point b with long periods of driving on small winding roads that only one car can pass ….so you have to feel comfortable not seeing what is approaching ahead - and using the mirrors they place on the road to see what upcoming traffic . Even experienced as an expat living abroad and driving all over Italy for years… it takes a little nerve to get around those corners :)!