All of your itinerary is beautiful, but you might want do give considering to the following things:
May is often considered "too early" for the Dolomiti, due to the fact that there is usually still a lot of melting snow around, and you can easily get "bad" weather (low clouds, rain, fog and even snow) that obscure the views. If you are ok with being flexible, the best approach is to book only reservations you can easily cancel on short notice, and have a "plan b" for going someplace else if while you are in Venice you can see that touring the Dolomiti that week would be a waste of time. money and effort. Some wonderful "Plan b" destinations between Venice and Varenna are the Verona area that includes Vallegio sul Mincio + Lago di Garda, or Bergamo + Clusone, or the small but lovely historic city of Mantova, surrounded by water and interesting small towns. Or just have a "Plan b" to stay in Venice for a few more nights if it is raining in the Dolomiti.
Also, is there a reason you want to stay in Bolzano rather than in a smaller town of village in the scenic mountains themselves? Bolzano is a small city, and while it is very small and historic and picturesque, if you are going to the Dolomiti for the natural beauty and have your own car, you might consider staying in one of the many beautiful lodges in the mountains with views, or one of many smaller villages with views that are on the way between Cortina d'Ampezzo and Bolzano.
If you do drive all in one day from Venice to Bolzano via Cortina d'Ampezzo and the Great Dolomites road, be sure to leave plenty of time to get to Bolzano, remembering that the sun sets somewhat earlier in the mountains. Not only is it likely that you will want to stop often to admire the stupendous views, but the drive is demanding mountain driving, so you will also want to relax and enjoy it, and have some breaks for a real lunch and coffee more than once, just to recoup before moving on.
The other suggestion I would make is that instead of using Sorrento as a base that you hire a driver in Naples to take you to one of the towns on the Amalfi coast itself, and sleep there and visit Capri from there. If you don't like steep hills and staircases, stay in the town of Amalfi.
Speaking purely personally, I can't stand drives of more than a couple of hours between destinations, so I would be trying very hard to get rid of that 6 hours of highway driving between Varenna and Montepulciano. Since nobody needs a car while in Varenna, what I would probably do it is to drop off my first rental car in Bergamo, and then take the train from there to Varenna. (it's a commuter train and the name of the station for Varenna is Fiumelatte.) To get from Varenna to Montepuliciano, I would take the train from Varenna to Milan, then the fast train to Florence, pick up the car in Florence and take a scenic route through Tuscany to Montepulciano. It would still be a long day of travel, and yes I'd be pulling my own luggage through more than one train station, but the reward for me would be less stressful highway driving -- especially if it turned out it was raining that day. But you say you are comfortable with driving, so maybe you would prefer just to do it all by car.