I would like to reiterate that if you go to Florence, unless you rent from Sixt rent a car, which is outside the station, for other rental agencies, such as Hertz or Avis/Budget, you will likely need to walk a bit on city streets as they are not currently at the station, although not a long walk away.
Of course you don't need to stop at Autogrill or any Area di Servizio if you prefer to exit the freeway, pay toll, find a restaurant in a town, then re-enter the freeway. However if you are planning to eat at a restaurant in the famed capital of good food (Bologna) driving into and out of the city will likely make your regret that choice. But of course it is your choice. I find the Autogrill (or Chef Express or Sarni) locations at the Area di Servizio along the freeway, to be very convenient on a long trip on the freeway.
If you ultimately decide to take a train from Bolzano to Firenze Santa Maria Novella, also you should know that you will need to change trains somewhere (either Verona or Bologna) as the only high speed train that requires no train change leaves at 1:12pm from Bolzano, which I think would be a bit late if you plan to leave Ortisei after breakfast. The journey by train would still be about the same duration as the drive, or maybe a bit less, but once you add the time necessary for returning the car in Bolzano and picking up a new one in Florence and find your way out of Florence to the Firenze Impruneta entrance to the Siena freeway, it would take longer. I'm not even mentioning the possibility of accidentally entering the ZTL in the Florence city center, or accidentally driving in the bus lane on either via Valfonda or via della Scala (both of which are enforced by camera).
Also, another consideration is that if you take a train, your lunch will have to be train food on the Trenitalia Easy Bistro, which is basically airplane food with a fancier name. Of course if you change trains at Bologna, you also could have the choice to get off the train, place your luggage at the Deposito Bagagli at the Bologna Centrale station, then walk to a restaurant in the city center, then come back after lunch to the station, pick up your luggage at the Deposito Bagagli, pay for that service, then take the next train to Florence. For that option you will need to have purchased different train tickets first for the portion Bolzano-Bologna and then for the portion Bologna-Florence because those high speed trains are like airplanes, with specific seat reservation for a specific train, so you can't get off and on any train at will. Therefore make sure the train reservations are such that you have sufficient time to go to the restaurant and come back on time.
In terms of savings, the cost of the high speed train for 2+ people would not be any less than the 28 € of tolls plus maybe another 60€ of fuel, if anything it is likely to be at least just as much for two passengers. In addition two separate shorter car rentals are likely to cost you more than one for a longer period.
I call all of the above unnecessary hassle for less convenience, more money and more time, just for the thrill of traveling by train. But of course it is your choice to follow the advice that your deem more beneficial to your likes and wishes.
