I personally would be moving around more. Your trip seems relaxing. I, personally, would have a longer list of things to visit, if I were spending 18 days in Italy. I would be hitting up more hilltop towns, the Cinque Terre maybe, or Amalfi Coast. I've always loved to find less talked about, but amazing locations, like Civita Di Bagnoregio. And I would be looking to try and find ways to replicate those experiences.
I absolutely love the scenery in Italy. For instance, the Renaissance Il Ciocco Resort, has amazing views from their rooms/pool deck, and all around the property.
I agree with the remark that the trip is hard to critique on the surface. I've never been to some of the locations you mention. Rome is great for that amount of time. I've never spent more than 1 day in Florence, and unless you appreciate renaissance art and museums, or have some particular tours planned; I would probably be bored of Florence by day 2 or 3. However, if you are using Florence as a base for Cinque Terre/Barga/Lucca/etc - then it would be no problem.