"DebbieAllen, it would help if you could tell something about your interests (art, architecture, which style/period?, shopping, beaches, hiking, landscape...)."

I'll do my best!
I am fairly well-traveled (Europe, Mexico, Asia, South Pacific) and prefer to avoid places that are mobbed with tourists -- in some cases that just means visiting off-season or early/late in the day, not crossing them off the list entirely.

My perfect travel day would probably be to wake up without an alarm and have a delicious breakfast -- either at the hotel or by wandering around the neighborhood and finding a little coffee shop or restaurant. The day would have one definite "sight" planned -- probably a museum, historical site, market, or even a cooking class -- and one definite restaurant to eat in, so everything would revolve around those choices. We'd head to whatever neighborhood or town our "sight" was located in and wander around before or after visiting, stopping to eat or window-shop when the mood struck. Whatever meal was not in the planned restaurant would happen on the fly -- though knowing me I'd have some possible spots in mind. If we had the option to swim I'd probably want to be out and about until about 3pm, then "home" to swim, sit in the sun, nap, have a campari, then head out to dinner. Or if we were at a beach location I might spend at least one day on the beach all day, then out after 5pm.

I love Italian art but will not want to hit sensory overload. My SO loves Roman history so he may likely insist that we visit Rome, though I believe there are other places that might give him just as much of a fix. I love warm weather vacations but don't feel like Italy has to be a beach-y trip (though Elba sounds lovely!).

When I come back I mainly want to feel like I had some of the best food of my life, saw beautiful things (whether a view or a painting), stumbled upon some amazing side streets, restaurants, people, and was charmed by the whole experience. (Tall order, eh?) I'm describing it this way to explain that I don't want to cross off a checklist of "10 most amazing sights in Italy." I'd rather visit multiple times and take it in small bites.

I like your travel style, and above all your reluctance when it comes to "must see" lists, plus your awareness that you have to come back anyway since Italy has so incredibly much to offer.
So what about reducing variety a little bit, in order to reduce travel time & effort, making this whole trip feel more relaxed? If you choose three locations in ONE of Italy's three basic parts (north, center, south), possibly splitting the large north into north-west and north-east, then you'd explore one part more in-depth, travel less, and relax more. This wouldn't mean to give up on variety altogether; you could still choose one small place, one town, one city, and mix seaside and inland locations. If you tell which of those four larger regions appeals to you most (in the light of this thread or else), and whether you are perhaps already set on any place there, you will certainly get more focused suggestions, by me and others.

This goes against your original thinking but you might consider choosing two home bases, six days each, and just take one or two day trips from there. Florence and Rome, maybe. That way you can get to know your home base city, but also have some great days on the water, in small towns, etc. Probably best with a car though.

I have only been once and am going again in October, would love to see more replies because I'm finding out about all kinds of little gems! Thanks all