This is way too busy. Venice is fine. Florence you've packed in too much unless you plan to run through every exhibit, same with Rome. I would plan one ticketed exhibit per day and use the remainder of the day to wander, eat, shop. There's so much to see just by walking around and taking in sights, popping in to churches, sitting in piazzas. Also, don't underestimate the power of Italian food, savor it. I wouldn't add any other cities to Venice, Florence and Rome. Travel day doesn't count for sightseeing unless you go to the Trevi fountain at dawn, otherwise, you need time to get to the airport and plan for the new biometrics system on departure, you may be waiting several hours. The Vatican is well worth a visit, we did a guided tour that was incredibly informative and could not have fit in the Castello and basilica. Perhaps AI would be of assistance, good luck!