Hi there!!! I am originally from Italy, although I moved to New York about 4 years ago. My BF and I had a similar trip to Italy last year, except that we stopped a few days in my beach house in the South for him to meet my parents.
I think the major budget issue in your plan will be Venice. DO NOT stop have coffee sitting in Piazza San Marco!!! I know that it might sound like something that will cost you only a few bucks, but those cafes went all over the news as they not only add a whopping 15-20 euros per person as “music surcharge”, but also completely inflate the price for tourists, so you can find yourself paying literally hundreds of euros for no absolute reason. They are scammers. Eating is OK, I would recommend the small, simple restaurants with fixed price menus, so that you know ahead how much you will be paying. If you go in the Rialto area (next to the famous Rialto bridge) you will find nice restaurant, with good food (including seafood) at good prices. For a fancy restaurant, go to Riviera. I do also recommend going to visit the Lido in Venice, you will love it.
Siena is lovely, I think it is one of the best choices for Tuscan towns. Make sure you take a few hours trip to Pisa to see the leaning tower. Siena itself is pretty inexpensive, and you will find a lot of food options quite easily. I ate in a couple of the restaurants in Piazza del Campo (right in the town center, where they race the horses in the summer) and they were all lovely.
Florence is wonderful, I lived there as a teenager. As of Siena, is mostly a historic place, with not too much to do at night (but you will be super tired after all the art and sightseeing). I advice you to get the ticket for the Uffizi, the most important museum, ahead of time online, as the line can be worst that the one to get on the Tour Eiffel in Paris!!! Firenze is NOT the place to eat fish, you gotta eat meat there. If you want to splurge the best place to eat, quite fancy, is the Enoteca Pinchiorri, right by where I used to live. Rent a bike in Florence, and please go the “Piggy market” (“mercato del porcellino” in Italian) to find some gorgeous handmade leather goods. If you are a fashion addict you must go right outside Florence in the factory stores of Prada, Gucci and so on. You will love it!!!
I do not know much about Garda and the Cinque Terre, as they are mostly places that we would visit on a daily trip, not actually sleep there. Feel free to PM me if you need any more advice!!! Btw… I do not know if this is your first time in Italy… But if it is, you should make some room for Rome!!!