I've modified my itinerary taking into account everyone said it was overloaded, and keeping in mind that I will make a return trip to Italy later on. Unfortunately my hotel rooms have already been booked a few months ago (I was overzealous) and can't be modified, so I can't spend another night in Venice. Here's what I have right now, doing my best to work with the days that I have to spend in Tuscany. @Roberto: I'll be dropping off the car on the 23rd in Siena before taking the train to Rome in order to save money.
16th, Monday (train): Train from Venice to Siena and explore home base
17th, Tuesday (train): Florence via train. See Academia and Uffizi and wander around city. I know 1 day isn't enough for Florence, but like I said, I plan to return to Italy at some point, and my main focus this trip is to explore the Tuscan countryside.
18th, Wednesday (car): pick up car in the morning and drive to San Gimignano
19th, Thursday (car): SR222 to Radda and Castellina
20th, Friday (car): SR222 to Greve and Panzano
21st, Saturday (car): Montalcino in the AM, back to Siena in the PM (husband will meet me in Siena instead)
22nd, Sunday (car): Crete Senesi to Montepulciano, then Pienza (will add Montalcino if there's enough time) and head back to Siena through Buonconvento.
23rd, Monday (train): Drop off car in the morning in Siena before taking train to Rome
Please let me know what you think, what you'd add/subtract/change. Thank you all for your valuable input and time.