Two more vegans here. We get apartments and go shopping at the outdoor markets. Even if you don't have a kitchen, I'd go to look and get some fresh fruits. With respect to your carbs (and Pam's potatoes), the breads in Italy were wonderful. Denis quickly developed a fondness for the pan a noce (walnut bread) at the Forno in Rome's Campo de' Fiori market.
In Venice: https://community.ricksteves.com/travel-forum/italy-reviews/vegan-in-venice If you're okay with some eggs (we "sinned"), a quick and easy choice is Dal Moro's Fresh Pasta, not far off Piazza San Marco.
In Florence we too ate at Il Vegano, mentioned above, which we did indeed find on Happy Cow.
In Rome we so enjoyed the groceries at the Campo de' Fiori market, including the Forno, which serves breakfast items as well as breads. I didn't record any other options in my journal, which means nothing to recommend there.
We rarely get to enjoy desserts, but we did get sorbetto daily. Most places marked their offerings with respect to allergens, so determining vegan selections was generally easy. If not, a simple "vegano", said as a question, did the trick. Enjoy some for us.
And for a different snack, in the cereal aisles, look for some spelt flakes, sigh, I miss those.
When I was looking for my post on Venice, I noticed there are other similar queries, which would net you more ideas.