If it were my trip and based on what you’ve already seen, I’d start with Assisi (2 nights) after you arrive from the airport (assuming FCO), then Siena (4 nights), and 4 nights in Montalcino. They are not far from each other, but a car is needed because public transportation between these cities aren’t great.
Assisi is beautiful and small, walkable village with a Basilica that is probably in my Top 3. It is also a little more than an hour drive to Siena.
4 nights in Siena allows you to have a day trip to Florence. If you like stifling crowds, stay in Florence. Siena’s Duomo is in my Top 3. Impressive. Siena is also enough to be walkable with books and crannies that are waiting to be discovered. I’ve stayed twice at the Palazzo Ravizza which is within the city walls and has parking.
Montalcino for 4 nights allows you day trips to Chianti and have a lot of time for winery visits. Il Giglio is a must place to stay. Great restaurant and wine list.
Depending on how often you want to pack and unpack, you can stay in Lucca which is in the northern part of Tuscany. You can visit the Super Tuscans in Bolgheri and the Tower of Pisa from Lucca.