You are already doing a great job planning this. What you need to resist is everyone adding more and more cities to your list, because they went there, and have warm moments of it. You don't need to add Cinque Terre, this, that, and the other thing. If you listen to all of that, you'll have 25 cities recommended that you visit in 21 days. Pretty soon, someone is going to recommend a day in Pisa.
As for Venice, it is a city to visit in the off-season. The question has been posed, "What is the most beautiful work of art ever created by humans." Nobel Prize in Literature winner Brodsky responded, "The most beautiful work of art every created is the city of Venice."
I'm not touting Venice. Brodsky even said that he would never go to Venice during the summer, even at gunpoint. I swore off Venice during the summer, but still found myself there this past July.
You could say the same thing about Florence. Only at gunpoint should you visit it in July. Sweltering heat. Super crowded museums where all you will be seeing if you can get within 25 feet of a famous piece of art will be 500 hands in front of you taking photos of it, or selfies.
Even though I avoid Italy in the summer as much as possible, i had to pretty half of the first six months of this year there, including Lecce, Matera, Naples, Rome, and Venice.
It is completely possible to enjoy Venice in July. You just have to remember two things; you have to stay as far as possible from San Marco Square; you have to stay as far as possible from the Rialto Bridge. Venice is called, La Serenissima, or most serene. Below are four pictures. The first two of them are what Venice looks like if you are on the tourist route, when cruise ships land. The second two were taken around the same time, about a ten minute walk from San Marco.
Venice isn't that much bigger than Central Park In NYC, but for some reason, everyone thinks that they have to congregate around Piazza San Marco or the Rialto Bridge. A couple of blocks away is empty. Just moms pushing strollers, the most quaint little shops to get a snack or a coffee.
Others may differ and I respect there opinions, but for Venice, the further you get away from the, "historic centers" of San Marco and Rialto, the better it gets. I had zero problems with the crowds depicted below when I was there this July.
Florence kinds of irks me because it has it's historic center, but unlike Venice, where the further you get from the center the better it gets, Florence's historic center is just as packed as the center of Venice will be in July, but if you wander off to try to stay outside, it doesn't get bette. It gets worse. Once you leave the historic center, which is quite small as in Venice, you will be in a neighborhood with one auto repair shop after another, for blocks and blocks. Used tire sale care lots. Stuff like that.
In Venice, if you happen to be dumped into San Marco, just get out of there. When you get far enough away, which might be a ten minute walk, stop into a church. You are likely to see a painting by Caravaggio, or Tiziano (Titan), or a Tintorello. 99% or the tourists going to Venice never see that aspect of the city. Florence has the same immensely crowded downtown that Venice has, but once you get away from it, you are more or less in a slum. In Venice, the further you walk away from San Marco the prettier it gets, the better the food gets, the more romantic it gets, and the more drop dead gorgeous it gets.
And it's way safer than walking outside of the historic center of Florence, which is basically a slum. Not so Venice, not only the most beautiful, but also one of the safest cities to visit. As long as you stay away from San Marco and Rialto, if you walk around the back streets, it will be the most romantic thing you have ever seen.
Putting that aside, you did a superb, fantastic job. One of the top 1%. Most feedback you get is going to be adding more and more cities. I suggest you don't go there, like adding a side trip to the Amalfi Coast from Positano
For specifics:
July 10 - 14: Florence
That would be a long time for me, because unlike Venice where there are normal parts of the city where you can escape to by walking 10 minutes, if you walk outside of the historic center, you are just walking to slums, and the historic center will be super crowded. Think about it as being at a major league baseball game type crowded, except the lines for anything you want to do in Florence will take you longer than it would take to get into Yankee Stadium.
July 14 - 15: Siena
Day trip from Florence. Take a day out of Florence time, take the train there, and take the train back in the evening. Not enough for two days, if you are both good on your feet as far as walking. And there is almost no place you should go to where you check in at 3PM, check out the next day at 11 AM. That's not vacationing. Even in Siena. It's just sightseeing and taking picture. Siena would be a nice day trip to break up your stay in Florence. And if super-crowded Florence gets to be too much for you, Siena would be a nice break, as would San Gemignano, the City of Towers, but between the two, I'd take Siena. Then get back to Florence. There's no reason to be moving around the much.
July 15 - 20: Montepulciano or Pienza
Montepulciano, but cut two days off it it, to add it to Rome. Or perhaps not. Spending 5 days in a small town, or better yet, stay in its outskirts, is the type of thing you should be doing on a honeymoon
July 20 - 26: Positano (split time with Ischia?)
That is a lot of time in Positano, unless you are staying at a luxury resort that will protect you from the immense crowds. There are 3 islands off of the Amalfi Coast; Capri, Ischia, and Procida. I'm 100% with you in blowing off Capri, or doing it as a day trip. Ischia is a little more normal. Procida, however, is normal. It's not set up as a tourist trinket shop sales place at all. You are spending a lot of time on the Amalfi Coast, and you can't spend 3 days of it in Ischia.
July 26 - 29: Rome
Rome is a big city. I spent most of this past July there, as I spent February there, and at least a few months of every year. It doesn't really notice that there are tourists. Like NYC, it's big enough to just absorb them. But 3 days in Rome doesn't really get you much. This is a city that has some of the best things to do in the world. Shorting it so that you are spending more time in Florence, Montepulciano, and Ischia, may be something worth reconsidering.
You are doing a great job. Just don't let anyone tell you to add more cities, and do the typical whirlwind tour. Your itinerary is find as it is. The above are just some feedback point. "Is Siena worth 1 overnight?" No place is worth one overnight. If it's that worth it then it demands more than one overnight, or it's just a day trip.
Splitting AC time between Positano and Ischia is a tough call. Positano is going to be super crowded, like Venice if you don't get away from San Marco. My choice of the three islands off the coast of Naples, in order, would be Procida, Ischia, and then way down the list for a July trip would be Capri.
a few concerns:
1. am i moving around too much? too many city/hotel changes?
2. is Siena worth 1 overnight - let alone 2?
3. thoughts on splitting the AC time with Ischia?