Ciao Okietravelere,
Well, Tuscany is a whole region so it does take time if you wanted to cross it all at one time..... luckily, many of the top visited areas are concentrated in this large circular area between Florence and Siena and that is about 1 hour car ride with all of Chianti falling in between.
Since you're planning to stay to the south of Siena, your "circle" is a little bit further down which includes all of the Val d'Orcia.... from Buonconvento, Pienza, the thermal areas in Val d'Orcia or La Foce, Montepulciano are all within an hour car ride, definitely doable. San Gimignano and Chianti wine region are also within an hour ride, with your own driving aroung the area being part of the fun of seeing the region and getting good views.
Saturnia and Talamone in Maremma are the only ones that are a bit further away and will take more planning.... you essentially have a mountain (the Amiata) in the middle between the Val d'Orcia and Maremma so the roads make you either go from Siena toward Grosseto and then south or south toward the mountain and over it, or south around it....
We've taken the road from Montalcino south towards Cinigiano and this is a beautiful panoramic, mountain road.....
I see Google maps says between Buonconvento and Saturnia it is 1hr and 40 min and makes you go through Arcidosso... I am not too sure about that timing, and if they really take the roads into consideration. Arcidosso is on Monte Amiata, they are nice roads but you aren't doing 70-90 kph on these roads.
Maremma is a large area, the roads once you're there do tend to be straighter which means higher speeds but pretty much from Montalcino until you're in Semproniano you're in the mountains/hills.
My personal recommendation is this: if your time is limited, take the bigger road between Siena and Grosseto (E78/SS223) to drive faster. If you have a chance, take the mountain road for a more panoramic, leisurely drive.... in any case, plan for at least 2-2.5 hr drive.
Or how about leaving this to the last day and stay in Maremma rather than drive back to Buonconvento?