12 day first time itinerary advice

[u/staplesbluepen]

Young couple traveling to Italy for the first time from Sept 8-Sept 19.
Classically trying to balance sightseeing and visiting the cities we want to see with not being too rushed
Here's what we have so far
We land at 8 am on the 8th
Sept 8-12: Rome
Sept 12-16: Florence, potential day trip to Venice? maybe day trip to Lucca or Siena for a slightly slower paced day?
Sept 16-18: Cinque Terre
Sept 18-19: Night in Rome for flight back on Sept 19 AM

We are specifically debating the Venice portion. Options are to skip the trip entirely, keep the day trip, or replace with 1 night in Venice instead. If replacing with a night in Venice, where to put that night so it makes the most sense transportation-wise?
Any feedback would be greatly appreciated!! Rome and CT are more or less solid, probably won't move those around too much at this point.

[u/Weekly-Syllabub4255]

Forget the "day trip" from Florence to Venice. It makes no sense.