If you want the pool and your dates are not yet set in stone, make sure it is in the earlier part of the month as by the end fall generally makes an entrance.... and places close their pools down by the start of October.
As far as places, I suggest the area in general because it is pretty central but leave the details to places you find that you fall in love with -- take a look at
https://www.tuscanyaccommodation.com/ to look at lodgings and go from there.
With a car, you'll see the area is actually not that wide or large, and that whether you choose Castellina or Panzano will not make much of a difference. The entire area merits exploring - and you will as you drive here and there.
But the place you choose will matter.
As far as the type of accommodation: keep in mind many villas are divided into apartments and you share any common spaces, such as the pool area, with other guests. Others are used as B&Bs and therefore are rooms, with common areas inside, like living room, with kitchen use generally not included.
Traveling as 2 couples, you might want to either have completely separate apartments or a large apartment with 2 bedrooms and share the common spaces or get 2 rooms in a B&B. Up to you!
Your search can be either by location and then by type, or start with type of lodging you want and then find an area to base yourself in. I find Siena a great base for exploring most of Tuscany, but the countryside between San Gimignano to the west and Gaiole in the east to be very romantic and emblematic of Tuscany so generally suggest staying in the area. If you decide on a type of lodging, I can suggest some specific ones ;-).
I also suggest planning on doing less and going with the flow... CT is not in Tuscany and would be a long drive and long day, if you want to include it. Map it on Google Maps to get a better idea.