First trip to Italy in July itinerary

You might consider taking a couple of days from Venice and adding them to Rome. Rome is such an important city that it deserves more time. 6 days in Venice would be too long for me.
When we went to Florence, we went in on a Friday and left on a Monday. We rented a car and moved south 1/2 hour to an agriturismo between Certaldo and San Gimignano for 4 nights. Virtually every farm in the region has rooms and/or apartments as a secondary source of income. And rambling from hilltown to hilltown is very pleasurable and an authentic Italian experience.

I assume you've already got your airline tickets lined out. With the advent of budget European airlines, you can fly cheap to just about anywhere in Europe. We like to take a trip to some place like Italy, and then catch a cheap flight to somewhere we've never been to for 2-3 days. And then fly home from there. It would be nice to reduce your Italy leg of your trip to 21 days, and then hit Copenhagen or Lisbon for 3 days on the way home.

With regard to taking days from Venice, don't take more than one. It is a truly unique place -- unlike any other we've been. A trip to Murano and Burano will consume the better part of a day. So will a day trip to nearby Verona and Vicenza. That only leaves three days for the city itself and some of that might be lost to arrival and departure times.

Your plan looks fine to me, balanced in terms of number of nights in each place.

Thank you so much. Very helpful suggestions.

It's hard to say re Venice # nights. If you plan trips to Padova, Vicenza, Verona then 6 would be required. Otherwise I'd take off 2 nights, one each for Rome and Vernazza.

Don't discount how much the heat will drain you. You may want to add a night or two to Rome because of that. You might even consider reversing the itinerary so you have a slower start while you're catching up on sleep and getting over jetlag. Rome is intense.

6 nights in Venice is a lot. Day trips to the islands are fine, but I wouldn't use it as a base for mainland day trips unless you're staying near the train station. Travel by water is a lot slower than you think it'll be.