Itinerary for Europe, in what cities would you spend more time

We will be there 13 days including travel days.

1.5714 days in each place. From which travel time between places must be subtracted. If you jiggle, that makes some places just a lunch stop. One day in Florence works for me. I'd want a wee bit more in some of the others.

Zurich is simply out of the way, and it'd be better if you just flew into Munich. Your itinerary is basically two separate trips. Getting from Vienna to Venice is somewhat difficult, and there are no inexpensive flights. The Alps block trains from that route, too. Have you considered modifying your itinerary? Munich, Salzburg and Vienna would be a good 10 day trip.
Venice, Florence and one more major Italian city (Rome) would also be a good 10 day trip.

If you have the energy for frequent stops, 2 nights in each place can give a good overview of each city. Arrive mid-day, scout around and get oriented; decide what is worth more time. The second day, go to the places you want to visit. Next day, get back on the train and go on. I would put any extra day into Florence, but that's me. Unless Sebersdorf is a "must see," why not go directly from Vienna to Venice? Some extra time in Venice might be nice. Two weeks is about the longest period of time I would travel at this rate. After that you would need a break.

Munich, Salzburg, and Vienna do very well together and would be a great 10 day trip. You could train between them and fly into one and out the other eliminating going back and forth. I would leave Italy for another trip.