I've been to Italy five or six times (only once via a cruise) and never tire of it! So so much to experience and see and ... eat! If you're going all the way over there, I think you're going to want to hit at least some of the must-sees in that amazing country.
That being said, we love that Rhapsody Croatia/Greece itinerary and are planning on it for next summer ourselves. If you really have the time/means to do Italy plus the cruise, I second (or third) the idea of starting your trip in Rome. Fly directly there (might even be less expensive than flight to Venice), spend 2 days seeing the main sights and start in on all that yummy pasta. Easy train ride to Florence, which is beautiful and easily walkable . . . with scrumptious gelato spots. Spend at least a night or two there if you can before heading to the wedding. The Tuscan countryside will be a perfect counterpart to the few busy days prior. If you eat meat, be sure to enjoy some Bistecca alla Florentine while you're amongst all those gorgeous vineyards. Then, as you're thinking, head on up to Venice via train and take in the wonder that is that incredible city. (Note: imho, food is not the highlight there, though we really loved our cicchetti food tour.) And then hop on Rhapsody to cruise the Adriatic! What an awesome combo trip that would be.
This would give you a great little taste of some of the main cities in Italy and likely will whet your appetite for a return trip (sorry -- not meaning to plan out your future vacations for you 🙂). Maybe then you can go explore the Amalfi Coast, Pompeii, other parts of Tuscany, eg Siena, and (only if you must) that tower in Pisa). And, because you will already have seen the main sights in Rome and Florence, you can just wander and enjoy those places in a less-touristy and less-hectic manner. Alternatively, since you'll probably be hooked on Mediterranean cruises at that point, you could do your next one in and out of Barcelona, making your way all the way over to Rome, still being able to get in more Italy along the way but, again, not feeling like you've got to cram things into single-day port excursions.
We personally have found that there have been several opportunities to visit Italy over the years (not all of them feasible) -- whether it's weddings or family trips or cruise stops or kids studying abroad -- and we hope to continue with those in the future, whereas Croatia/Montenegro/Greece (again, for us) are a little less mainstream and likely will not be done as often (though we are looking at our cruise as a precursor to more Croatia later). If you are able to swing the "Intro to Italy" plus the more "exotic" cruise, go for it! Looking forward to reading about what you decide.