Itinerary Suggestions - Germany-Austria-Italy

That is a lot of moving around as well as a lot of time traveling from place to place. Looks like you have 13 nights total? I would concentrate on Germany and Austria and maybe a dip into Italy (a popular favorite!) in the north and drop Tuscany.
We did a similar trip (Munich, Salzburg, Cesky Krumlov, Venice, and Verona) within a two week time period and it was definitely a lot of time spent in transit.

I have been to all the places that you mentioned, except Volterra.

I think your plans call for short visits, except for Salzburg. Still, you only need a full day in Rothenberg. You need more time to see Nuremberg.

You plan to head to Venice on the 28th, A train will take more than six hours to get there. Consider the night train.

Clearly, you have only a day in Venice. Way to short of time to see that wonderful city. Have you planned what you will see there?
Siena you can see in a full day. Be sure to climb the bell tower in the square.

Day train to Venice. Enjoy the scenery.

Day trip from Salzburg - ice cave at Werfen. Salt mine at Hallein unless you did the one in Hallstatt

First of all, what is your mode of transportation? And where are you flying into and flying out of? Have you bought your flights? You may be taking on too many locations.

If you're going by train all the way, your trip is going to be difficult. You're going to be close to some great cities but all together missing them.

A better itinerary might be:
Fly into Munich and stay there 20-21-23. (Great palaces and local sights.)
Take a train over to Salzburg (2 hrs.) 24-25-26
25th--take a day trip to Hallstatt
26th--take a tour down to Berchtasgaden
27th Take train down to Innsbruck, thru the Brenner Pass and down to Venice
Venice 28-29
30 Train to Florence
31 Florence touring
1 Take organized tour of Tuscany--hopefully hitting Siena, San Gimignano and Volterra
2 Fly home out of Florence

I kind of like it. A lot of moving but not long train trips. Maybe cut a day in Rothenburg and stay 2-3 nights Salzkammergut.

I'm going to add that the corridor from Innsbruck to Venice is totally awesome. I explored it last summer, and I'm going to come back for a couple years straight I think.

You could do a lot worse than just planning your trip to make stops along that route.