I spent 2 weeks in Italy. We arrived in Venice and went by train south to Florence and then Naples. Then we came back up to Rome, and spent a whole week in Rome before flying home.
If I was to do it again I’d save Venice for last. I was irritated by the traffic in Florence after enjoying the peacefulness of Venice for a few days. Plus Venice has wonderful glass that you might want to buy, and it’s best to save that kind of shopping for the end.