I'd change the itinerary around. It's less travel time to go from Rome to the Cinque Terre, then to Florence/Siena and then to Venice. As for the time allocation, I'd spend at least one more night in Rome - assume you'll be jetlagged on arrival so even after your first night on the ground . . . and for your first full day . . . you may be in a haze. Rome has lots of major sights and even 3 full days isn't a lot, especially if you're going to use a 1/2 day for Ostia Antica (recommended). If you aren't interesting in hiking, 3N in the CT may be a lot. How about 2N there and 2N in Siena? For 1-2N between Florence and Venice, you could stop in Ferrara, Bologna, Padua or Verona. But I'd probably opt for another night in Rome. So Rome 5N, CT 2N, Siena 2N, Florence 3N, Venice 3N. Slow down and sip the wine. Less is truly more in Italy.
Instead of changing hotels for your last night in Venice, you could stay walking distance to the train/bus stations and take a land taxi to the airport if the first bus out is too late for your flight. It's not the most desirable part of the city, but it's perfectly fine and all of Venice is not that big, so you are never far from anything.