Tuscany/Italy itinerary advice needed

Are we overextending ourselves? I’ve heard how lovely it is to stay overnight in Siena, but is it just too much? Better to just stay in Florence and day trip?

I think CT is too far for a day trip even with a driver. The rest looks good.

Florence has soo much to offer. The Accademia Gallery, location of David, is best done with a guided tour (in my opinion) and will take 1/2 a day. The Pitti Palace and Boboli Gardens can take the better part of an entire day if you let them. Additionally, there are plenty of stunning churches, museums and other palaces to keep you entertained for days on end.

As for the countryside, I would heartily recommend a wine tasting/vineyard tour of Chianti. And while I wasn't too impressed with Pisa (mainly due to the oppressive crowds), Lucca is a wonderful day trip. Both Lucca and Pisa can be combined into one (self-guided) day trip (via the train) but I would do Pisa as early in the morning as you can in order to avoid the crowds.

Your CT trip is going to make for a long day, and you'll miss the quieter morning and evening hours in the villages that are really the best part, but if you leave early enough it's do-able.

Rather than spending just one night in Siena, you might consider a day trip. Then you could get an early start to Rome from Florence the next morning. As you probably know, you'll be in Siena just three days after the Palio; I imagine they'll still be talking about it.

I loved Lucca; it's not a blockbuster-tourist-site kind of place, but it felt like genuine, lived-in Italy. I met several Italians who had moved there from Rome or Florence and preferred the more relaxed lifestyle. And you could definitely see Lucca and Pisa in one day. If you just want to see the Field of Miracles, Pisa doesn't require that much time. The reply above mentioned going there early to avoid crowds. Late afternoon/early evening also works well; the buildings stay open late and most of the day-trippers are gone.

Regarding spending hours in art galleries, I wasn't too enthusiastic about going to Florence for the same reason - people talked about the art, the art, and the art, and it didn't excite me. But Florence turned me into an art lover. There's so much beautiful work, and not just in the museums; the frescoes in the churches are mind-boggling. Plus, there are plenty of places to visit beside the museums.

I would recommend Lucca rather than the Cinque Terra. I stayed 3 days in Lucca and loved it.
But if you only have 1 day, rent a bike, ride around the city walls, climb a couple of the towers and enjoy wandering around Lucca.