We just returned from some of your proposed cities. I do need to write a Trip Report, but a few initial thoughts-
We love Lake Como, have been to Varenna before, but it's getting too crowded, just like the CT and Amalfi. Sadly, we won't return. The regional trains and ferries can't handle the crowds, (basically don't try), and there's no shoulder season anymore. The influencers seem responsible, and it's intolerable by mid-afternoon. It's also getting expensive and difficult to find reasonably- priced accommodations. I'll attach the previous threads. This is all pretty sad.
Avoid at all costs the holidays and weekends around them. We ran into May 1 ( Thursday) holiday crowds, which extended into the weekend. Also avoid the full week before and after Easter. I believe April 25 is also a holiday- the Italians have many.
We love Venice, and stay away from St Marks Square, so don't experience the crowds. We love the Cannaregio area, and Hotel Ai Mori d' oriente, on the quiet back canals. Will return again. I would stay four nights here. Many options for day trips out of Venice.
Not a big fan of Milan, except for flying into/ out of and sleeping over. Hotel Berna is great for this.
We used Milan Linate airport, in town, with much easier access than Malpensa, which is 45 min express train out of the city center.
So- maybe Rome, Florence, Venice, and overnight in Milan?
Have a great trip!
https://community.ricksteves.com/travel-forum/italy/is-lake-como-getting-crowded-like-the-ct-and-ac
https://community.ricksteves.com/travel-forum/italy/private-transfer-from-varenna-to-milan-centrale