I don't think that the Chiusi thing will be faster than the direct bus which links Siena city center with Roma Tiburtina in about 3 hrs.
But it's an excellent idea to stop over at Chiusi and to visit the famous Etruscan museum and the Etruscan tombs.

BTW:
the journey time of the direct regional train from Siena to Chiusi is 1 1/4 hrs (fare: 9 EUR).
Chiusi - Rome is 1 3/4 hrs by regional train (fare 10 EUR) and 1 hr 40 min by Intercity (fare 21 EUR).

ah yeah always forget about buses like Siena-chiusi-Rome - regional trains are slow so buses can be faster and always cheap.

We have a rental car to drop - so it’s either drive Siena to Florence airport and take fast train from SM Novella (only 1.5 hr train) or drive Siena to Chiusi to drop for regional train (2 hr) no s”reserved seats/no 1st class

but getting from Florence Airport to SMN station could take a while - recent thread talked about returning car at SMN itself. There are also some car rentals on edge of Florence center. Train from SMN would certainly be more comfy that regional trains which can always be crowded. If you could turn car in at Siena you could take short cheap bus right to SMN station.

I think everything has been said:
cheapest and fastest solution: drop off car at Siena and take direct bus to Rome,
if car drop off at Siena is not possible, drive to Chiusi and go on from there by train.
If you want to maximize journey time and money waste, go via Florence airport.

Well how about dropping car in Siena and taking bus to Florence SMN and get the fast nice train and be in Rome before that slow dumpy regional train.