Congratulations and Happy Anniversary!
I agree with all of the above. Your biggest challenges will be heat, crowds and time.
If you can add some days always go with the longer trip.
I am going to assume 20 nights- if you count your tip in nights you’ll have a better feel for how the itinerary flows.
Avoid 1 and 2 nights stays. Every time you move location you lose at least half a day. Sometimes 2 nights is really all you need in a smaller but do avoid as possible.
Have you determined your best flight option? Look for “multi-city”, “open jaw” fly IN to one city, out of another to elimnate backtracking.
Often best flights are in/out of Rome and/or Milan.
Do you have a great interest in Milan? Last Supper and Duomo are worth it but otherwise I think Milan is skippable - assuming no specific interest on your part. You’d have to be willing to commit time/effort to getting tix for last Supper- they sell out fast and early.
OK minimum stays in my opinion
Milan 1 or 2
Lake Como 2 or 3
Venice 3
Florence 3 add a night for every planned day trip
Rome 4- 5 is better
Amalfi/Naples 4 or 5- it takes time and effort to get to and from
If you can fly into Milan, out of Naples
Lake Como- 3 nights
Venice 3 nights
Florence 3 nights
Rome 5
Amalfi/Naples 5
That’s 19 nights- add to Rome or Venice
Entire itinerary easily done by train although you might want/need a private drive in Amalfi.
If you skip Milan and would consider renting a car for a few days- fly IN to Venice, OUT of Rome or Naples
Fly into Venice- easier to fly in than out of and perfect place to get over jet lag
Venice 3 nights
Florence 3 nights
Pick up car- Tuscany hill town of choice- 4 or 5 nights- this is a honeymoon after all!
Drop car Chiusi or Orvieto
Train to Amalfi Coast- 4 nights
Train to Rome- 5 nights
That’s 19 or 20
OR if you don’t want to rent a car
Venice 4
Florence 4 or 5 w/ 1 or 2 day trips
Train all the way to Amalfi
Amalfi town of choice- Sorrento- although not ON the coast makes a good base and with this year’s transportation problems in the area makes good sense
Sorrento 5 or 6 nights - day trip to Naples unless you really want more time there
Train to Rome 4 nights
Fly home from Rome
Skipping Amalfi- again due to crowding/lack of accomodations, difficult transportation
Fly in to Milan
Lake Como 3- also good place to get over jet lag
Venice 3
Florence 3
Tuscany hill town- with car- 4
Rome 5 or 6- you can never have enough time in Rome and if Pompeii is important you can day trip- brutal but doable.
Fly home from Rome
Anyway lots of combinations.
Leave your departure city for last so you don’t backtrack or split stay.
You might also consider the Dolomites instead of AC region.