December travel in Tuscany

First I'd look into car selection to fit six adults + luggage unless you were planning to rent two cars. All drivers need a IDP and be sure to familiarize yourself with ZTLs if you have not driven in Italy before. At some times of year, you must have tire chains--just something I have read about.
There is an overwhelming number of options, and I think your choices are fine.
Saturnia is a two-hour drive--not worth it to me, but there are other hot springs so I'd look for something closer. Hopefully one of our Italian locals will chime in.
https://mytravelintuscany.com/the-wild-natural-hot-springs-in-tuscany/
For wine, you'd probably need a driver and I'd look into organized tours, or just huddle in a warm tasting room instead!
I highly recommend googling "winter in Tuscany" and similar to get some specific ideas--good luck!

These are the dark days of winter - only a few minutes more daylight than Seattle. The weather can be pleasant with warm sunny day, and the weather can be cruel with cold rain and even some snow. The smaller cities will have modest street activity and the small towns none. Stick to the larger cities Florence, Siena, Lucca, Pisa. Many wineries will not have the usual service, though many that offer dinner will be open.

If hot springs are high on your list of things to do you might consider the short train over to the pretty little spa town of Monrecatini Terme, about an hour or so via the Regionale from Florence. There are a number of thermal spas there that cater to every taste and budget, and a few of them date back more than 100 years to an era of unparalled elegance. One in particular, Terme Tettuccio, apparently dates back to the 18th century and looks like something from Roman times. We peeked in while we were there and it appeared that the cost for day use wasn't particularly exorbitant - might be what you're looking for.
Might add that just above the thermal area is Montecatini Alto, accessed via a scenic funicular and which is a fascinating little place to walk around and perhaps have lunch at one of the many restaurants which feature spectacular views of the countryside below.

A few thoughts about Saturnia. We stayed in the Maremma for a week, and spent a wonderful afternoon in the nearby springs, in May. This is a very long way from Florence. The water was warm, not hot. In December/January I think you might FREEZE!!

thank you all for your responses. I think we have decided to save Tuscany for a warmer time and head to Rome instead. I appreciate your help!