There is always the temptation to cram everything into a trop like this, especially when it is your first time. There is a fear that you will miss seeing something and never have a chance to see it again. My wife and I have done both the "if it's Tuesday, it must be Belguim" kind of trip as well as trips where we slowed down a little and explored an area in more depth. We tend to prefer a less hectic pace. Our dream is to spend 3-4 weeks in Rome alone, with most of the time not doing touristy things.
That said, I could make an argument for you to either keep or discard Venice on this trip. If you decide to keep it, do so only if you can spend a minimum of 2 days and three nights there.
Venice and Rome especially have been overrun by cruise ship day trippers. The only way to enjoy them is to explore the out of the way places while they are there and see the main sights early and late, before and after the thundering hoards have come and gone. It's a question of both quantity and quality. It would be bad enough if the masses were considerate and polite. Unfortunately, there is a significant percentage of the cruise ship crowd that are rude and obnoxious.
Since it looks like you are already loaded up on cities, I suggest you consider substituting the time you would spend in Venice with spending time out in the Italian countryside, exploring rural areas and small towns. There's a completely different rhythm and atmosphere that is just as much a part of Italy as the major tourist cities. Either add a day or two to Varenna or find a smaller town in Tuscany.