Travel time is gonna make this schedule hard. Plus, it's hard to see any place in only a day and feel like you were actually there.

With that said: Of the places you mentioned, Venice, Milan, and Athens are 1-day candidates.

Mainly about Venice -
Everyone that has never been to Venice feels compelled to go. However, most people that have been to Venice, don't feel compelled to go back.
You'll see recommendations to skip Venice and spend time elsewhere.... and they're not wrong. But, it's a hard thing to do. I think most people should just go ahead and go to check that place off the list, but don't spend more time than is necessary.
Now, there are some cool things to see in Venice. St Mark's and Doge's Palace are great.
And there are other things to fill up a day:
Rialto Bridge, walk around some, and have lunch/dinner overlooking the South waterway. Gondola ride if that's your deal. Maybe hop over to Murano/Burano.
But.... there's not much else to see here where I would trade an extra day in Venice for a day somewhere else.

If you do shrink Milan and Venice to 1 day each, both of those days should be allocated to Florence/Tuscany.
I could give you a whole week of things to do in this area.
Florence is easily 2 days, with a 3rd not being wasted. And Tuscany deserves 1 day At a Minimum! If pressed, I would split it 2 & 2. 2 days in Florence, 2 days in Tuscany.

Rome/Vatican is 3-4 days. Can't argue with the 4th day, but it can be done in 3.

Not sure what your plans are for Amalfi Coast. Sorrento, Positano, Capri, Ravello, Amalfi, Salerno... one, all?
Positano is #1 in my book. Capri was a cool day trip.
After a week of walking and sight-seeing, having this stop in the middle will be a welcomed respite.
3-4 days is about right, but once you leave, you'll always want to go back.

Not sure how you're travelling... I assume trains.
If by car, here are a few cool stops in Tuscany:
Siena, Montalcino, Montepulciano, & Orvieto.
Buy some wine & ship it home! Also lots of local farmers/families that'll serve lunch.

I haven't been to greece. So, you'll have to get feedback from others on that.
Enjoy your trip! Sounds super fun!