Ciao MaggieMay, welcome to our forum!
I would suggest maybe dividing up those 10/12 days in maybe just two or three places, and do day trips to explore the part of Tuscany you're staying in. I think it would be less tiresome to be checking in and out of places every 2 days.
Unfortunately, some towns are set on hills and would make it harder to visit, although not impossible. Siena might be hard because it is spread out over various hills, but it is doable if you know which streets to take - I'll give you more info on that below.
I would suggest from Florence to head to Pisa or Lucca, make one of those your base for 3 days. Visit both towns, both are small enough that you don't need to walk around too much to see the main sights.
Then head to Chianti, I suggest Castellina in Chianti as a good central base for 3 days. Many towns in Chianti will be off limits since they have parking below and require a climb (Radda and San Gimignano, for example) but you can visit Greve, Castellina and Gaiole without problems because they are small and the parking lots and the main village/center of town are at the same level so no hills to climb. You can drive around the entire area and enjoy the views, stopping at wineries when you want or restaurants off the road that don't require any walking or climbing.
Then head down to the Val d'Orcia for the last 4 days - I would suggest staying around San Quirico d'Orcia. You can visit Buonconvento, San Quirico, Pienza, Bagno Vignoni without a problem, no hills.
You can visit Siena while you're either in Chianti or Val d'Orcia, maybe from Chianti would be easier as the drive would be shorter.
You can avoid hills and too much walking in Siena as long as you go directly to the Santa Caterina parking lot - https://en.sienaparcheggi.com/en/1028/"Santa-Caterina"-Car-Park.htm
It is right below the cathedral/duomo which is at the top of the hill but it has electric escalators that get you up most of the way. https://en.sienaparcheggi.com/en/1033/“Santa-Caterina”-escalator-–-western-docking-Area.htm
The escalators are not right by the parking lot and do require a bit of a hill climb to get to them - you can see the climb from this shot:
- the escalators are off to the right just beyond that door/gate you see in the walls up ahead.
If you don't think your husband can handle that walk, see right there to the right the yellow markings on the ground? that's the bus stop right outside the exit of the parking lot and the S54 bus goes to Piazza Indipendenza which is very close to Il Campo. You want to see Il Campo (does require some steps down into the piazza and back out) and the cathedral/Duomo. If you follow Via di Città to Via del Capitano and turn right, you have a climb but gradual to the Duomo (you can head to the Baptistery first by taking via dei Pellegrini to visit that but then go back to Via di Città to get to the Duomo. The shortcut by the Baptistery means a lot of stairs to get to the Duomo).
I think these are the minimum things to see in Siena with minimal walking and hills - you can catch a taxi back to the parking lot if by the end your husband doesn't want to do any more walking or hills. I suggest you follow the "Street View" along the streets I've mentioned above to see what they look like as far as the "hills" and steps you'll find, it will let you decide if Siena is a place you can both manage.
Also consider whether you want to bring a walking cane/stick that could provide some support because may streets are uneven, even when they are not cobblestone. You might also want to look into renting a wheelchair while here, if it would make it easier for you to get around. The Misericordia in the center of Florence has some it offers for free, they are not motorized, but you can check them out while you are in Florence to see if they could provide assistance or not.... with cobblestones on some streets, it might not be a great help in the end.
Let me know if this info is useful and post any other questions you have, I'll try to help!