Quite doable and a very nice trip.
The only potential flaw will be your flight time from Venice.
Consider a driver from Naples to Positano. If you splurge at all that is a good place to start.

The only change I would make is to train from Rome to Florence (90 minutes on the bullet train). Pick up your car in Florence and drive back to Siena if you want to base there (about an hour drive from Florence). Then you can drop your car back in Florence and pick up the bullet train to Venice. (It can be very difficult to drive out of Rome.)

All doable. Weather should be lovely. Have a great trip- Italy is spectacular!

Sounds like a nice trip!

A couple of thoughts-

You might want to consider staying outside of Siena when exploring Tuscany. Have you considered staying in a small town or an agriturismo in the countryside south of Siena? It makes the daily drives much easier than trying to navigate out/into a city like Siena each time you get in the car. Plus, you'll be in cities the rest of the trip. Staying on a beautiful property, surrounded by lovely landscape and sunsets- it might be a rewarding and relaxing addition to your trip. Take a look at places near Pienza or Montepulciano.

Also, you might want to just keep the car and drive to Venice and drop it there. Trains from Tuscany to Venice might require some changes and take more time than driving.

Have fun!

Thanks bobthenabigato. Do you have any idea as to the cost of hiring a driver to get us from Naples to th ac? Sarge56 thanks for your suggestin... Makes sense. We are not stuck on making Siena our base. I will look into finding smaller places and zoecat's idea is appealing. I will be back for more help once I have done Some more research.

I agree take the train from Rome to Florence and pick up the car there and head out of town. We stayed at a lovely villa outside of Sienna about 10 years ago and loved it. Was close to Sienna but you weren't bogged down in traffic as another poster mentioned. It was easy to drive around to all the hill towns from there as well. The train from Florence to Venice was easy as well we didn't have to change trains. We hired a driver from Naples to Positano about 7 years ago and it was a great splurge so you could sit back and take in the views without having to pay attention to driving. I believe it was about 150E at that time. If I were you I would drop a night in Positano and Venice and add 2 nights in Florence. We have been all over Europe and it is our favorite city. Have a wonderful trip!

NWheel, it looks like a great trip. Staying in Positano is a wonderful start - one of my favorite places in Italy. A driver from Naples to Pos will cost between $100 and 125 euros. Worth every penny IMHO.