We had a similar itinerary to yours a couple of years ago. Took the train from Venice to Florence. We decided to skip Florence on this trip and walked from the train station to the Car Rental (it was very close). Stayed 3 nights in a farmhouse in Tuscany (the only way to go in my opinion!) and then drove to Orvieto and again walked from the car rental to the Train Station. Had a nice lunch and toured Orvieto before catching the train in to Rome. It was really seamless. IMO, I would stay at least two nights at the same place in Tuscany, rather than packing up and moving after one night. Have fun!