Positano (and Sorrento) book up fast. You should choose your hotel now, or you will have your sixth choice. Parking is virtually nonexistent in Positano, so if you insist on driving yourselves, you need a hotel room that includes a private parking space. You can expect annoying delays as busses attempt to round sharp curves (blocking) the two-lane road that is the only way in and out of Positano. I'm not talking about a few curves and poor guardrails that mean nothing to guys with driving gloves and curved-heel driving shoes made from unborn calves leather ... I'm talking about rush-hour like congestion on a road that's already slow-moving.
Have you made some travel time estimates? It's quite a long ride from Florence to Naples. I would certainly not rent a car inside the city of Naples. And there is little to gain by driving from Naples as far as Sorrento. Since it is "beyond" your coastal plans, and because you don't express any interest in the city of Naples, perhaps you should take the Trenitalia to Salerno and drive from there to Positano. That, unlike from Naples to Sorrento, would be a scenic drive.
I would choose Venice over the AC for a first trip to Italy, but that's entirely up to you. For travel time on a very short transatlantic trip, it would make even more sense to do Rome-Florence-Tuscany. (I haven't been to Cinque Terre yet, but it could be said to pair better with Florence than does the Amalfi Coast. It doesn't pair with Venice well.) It is always a long hike out of Positano, but since you're not flying that day, you have it right. You might reflect on whether you want to waste potential tour time staying at an airport hotel, or fly out late enough to sleep in downtown Rome that night. (Not arguing either way. It's just that many people staying in Positano don't realize how "remote" it is.) There has been a direct flight from Naples to NYC, but I've never heard of the airline.
For planning purposes, it's helpful not to use a term like "3 days". Starting with an itinerary line like "Toronto-Rome (1-night)" puts you on the road to really counting out the trip, and not imagining more touring time on the ground than you will actually have. You are completely losing a "day" between Florence and Positano, for example.