Is it a good idea to have a local travel guide drive us from Florence to Siena so that we can make stops on the way?
It's only about 70 km. Where would you want to stop enroute? Are you changing accommodations, or doing a day trip to Siena?
Changing locations.
There’s a bus (also a train, but not quite as convenient). From either location, there are guided tours you can take - Tours by Roberto is highly recommended.
In Italy used a service called "My Day Trip" when we wanted to make stops while changing locations. (We've used them in other countries, as well.) It's not a guided tour, it's just a driver, if that's all you need. https://mydaytrip.com
Thank you!