I agree that Milan doesn't need 4 nights for most people. I'd put Venice way ahead of Milan for a first visit to Italy. Note that Sorrento books up six to nine months in advance, so you should not delay making that decision. Of course, El Nido and Antiche Muro are not the only hotels there (although you would think so from reading this newsboard!) We stayed at the huge Ambasciatori, on the cliff facing the Gulf of Naples, because it was our fifth hotel choice. A little expensive, but massive, lovely modern rooms, frigid air conditioning (desperately needed the last week in May), and an elevator that leads to a fixed (not floating) gulf-bathing platform, private to hotel guests. Wood boardwalk, not "beach". But we paid extra for a balcony with a plunge tub, and motorized window shades facing Mt. Vesuvius every morning. And some slight seagull problems on the balcony! We might have had a second TV in the bathroom, I'm not sure about that!
Your selection of Sorrento is correct, for two nights and good transportation connections. Note that Sorrento is not a beach town, and the pebble beaches in Positano aren't that great, compared to the U.S. Atlantic Coast or the Caribbean. You are right to move to Naples for a 6AM departure. There is nothing village-like or pre-war about Sorrento, but it's relaxing and scenic. You didn't mention Pompeii, which seems odd. It's a landmark of Western Culture.
Every day we walked by the Circumvesuviana station in Sorrento, there were more people in line for the SITA bus to Positano/Amalfi than would fit on the next, half-hourly departure. So we hired a car and driver (through the hotel concierge) to take us to see the Amalfi Coast. Since I "had to see Positano", but it turned out to be underwhelming, this worked out great for us. But it's expensive. The ferry is much cheaper, but note that Positano can only take a smaller boat, which means less availability. I did think Positano was more scenic than Amalfi, but Ravello beats them both, especially if you don't need to Instagram the same miserably expensive clothing and cosmetic boutiques you can not-buy-anything-in in Rome or Paris!
https://community.ricksteves.com/travel-forum/italy/getting-around-amalfi-coast
https://community.ricksteves.com/travel-forum/italy/naples-to-sorrento-train-or-private-driver
https://community.ricksteves.com/travel-forum/italy/amalfi-coast-for-7-nights-itinerary-help
https://community.ricksteves.com/travel-forum/italy/herculaneum-and-pompeii-questions
https://community.ricksteves.com/travel-forum/italy/pompeii-and-vesuvius-69e8366c-1e75-4a14-a0bc-0677269492d1
https://community.ricksteves.com/travel-forum/italy/amalfi-coast-for-5-days-where-to-set-base-from
https://community.ricksteves.com/travel-forum/italy/one-week-on-the-amalfi-coast
https://community.ricksteves.com/travel-forum/trip-reports/trip-report-2-weeks-of-day-trips-from-sorrento