I was exhausted after reading 3/4's of your itinerary. One of the above posters mentioned your itinerary reflected more time getting there and less time being there, which was a cogent observation. I would add more time to Roma, Florence, and Venice and drop Le Cinque Terre, Assisi, and Siena. You could see Orvieto in an afternoon while on your way somewhere else.
Sorrento and the Amalfi could be worked in, but at the sacrifice of some place else. Roma is immense and giving yourself 1.5 days, and one of those days being an arrival day, is not a good strategy.
Lastly, flying home from Venice is not the best plan because many flights leave very early in the morning before public transportation is operating, requires expensive private transfers. If it were me, we would be starting in Venice and returning from Roma.
Italy will seduce you while you are there and you will want to return. So, plan on returning to see some of the places suggested you drop this time. Enjoy a vacation and not a marathon at a dead run.
Buon viaggio,