Request tuscany itinerary suggestions

We have a 21 day trip to Italy planned from end of April through mid-May. We are very happy with time and places planned for Lake Como and on the Riviera as well as the six days in Rome at the end of the trip.

I'm interested in securing suggestions for the time in the middle of the trip when we are spending 3 nights in Florence, one night in Volterra, two nights in Montepulciano followed by two nights in Siena. We will pick up a car at the end of our time in Florence and return it when we arrive in Siena.

My questions include: Are we spending too much time in that area? Are we spending our time in the right towns? Should we try to drive to Umbria to see Orvieto?

Thank you for your observations.