advance tix worth it 9/17-23 for Milan Cathedral,Uffizi and Accademia,Basilica/Doje's Palace?

I would buy tickets in advance for the Uffizi, Accademia, and Doges' Palace. Unless you consider leisure to be standing in long lines waiting to buy tickets.

You won't have time for much in Milano even if your flight is on time. The train into the city takes about an hour. You can buy tickets for Leonardo's Last Supper from a broker at a markup, but still far less than a tour would cost. But if your plane gets in at 8:20, it will take you until about 11am to get to Milano Centrale. You could leave your bags at the train station and then take the Metro to the Duomo stop, see the Duomo, perhaps go to the roof, and get back to the station to grab a takeaway lunch and pick up your bags.

The Last Supper visit would take an additional hour, including travel time by taxi to/from the Duomo or Milano Centrale. I use Select Italy for tickets, which include a markup but not a tour.

You are traveling to Italy in high season. Buy advance tickets for every place you want to visit.
Venues were crowded in late October last year.

You now have to get a ticket to enter the Cathedral in Milan and the line can be quite long. All the places you mentioned will still be busy. Florence is busy every time I go there and I have been there in just about every month except August and December.

I think you're mixing up a couple of destinations and deals.

You definitely need to reserve Florence's Accademia and Uffizi if you don't want to spend a hefty €72 per person for Firenze Cards that you don't have enough time to use to full advantage. I'm not aware of a one-day combo ticket for this pair.

There are other ways to avoid lines at Venice's Doge's Palace, such as: Arriving early (they open at 8:30); or buying the Combo Ticket across the piazza at the Correr Museum, which is also included and need not be visited in the same day; or buying a more expensive museum pass but you again may not have time for more.

The reservation fee for St. Mark's Basilica is a measly €2 that you can throw away if you change your mind, or that's the church that has the shorter-line benefit for people who check a bag (it's not in Milan).

Yes, the Last Supper in Milan is important, and impressive, and popular, and deteriorating. I think it's perfectly practical to skip the reservation hassle of this short visit and focus your attention on the simpler and also impressive sightseeing around the Duomo. You can see DaVinci's Annunciation and his Adoration of the Magi in the Uffizi.

You can buy the Milano Duomo ticket across the street but the line can be very long. Buying some tickets online ahead of your visit might be the most efficient thing to do, even though you are committed to a date/time.

Thanks everyone- this is why Iove the RS forum!
Indeed, I was confusing the tip regarding bags at the Milan Duomo with the venues in Venice. We'll have our hotel in Venice so won't need to take advantage of the bag check there.
Regarding checking our bags into Milano Centrale before heading to the Duomo; does anyone have any recent experience with how long it typically takes to get through that line, both to check in and to check out? I have tried to conduct my own research and it seems to be fairly common that the lines are long though some say it moves pretty quick.