Best train service from Venice to Milan

Is it just a day trip or will you have luggage to be dealt with?

Only Trenitalia will get you the earliest. If you have luggage, it will take time to store it at the station. You can the take the Metro to Duomo in a few minutes. M3 (Yellow line)

Do you mean you will depart Venice from the Venice train station (Santa Lucia) for a 9:30am Duomo tour in Milan? If you get to Milan Centrale by 8:15a, you have plenty of time to get to the Duomo. You will catch a Metro train from Milan Centrale (the trains are on an upper floor; you take an escalator down to the metro station, buy the tickets you want, and follow the line you want). The metro lines are organized by color. [Edited out color of metro you need]. The metro takes about 10-15 minutes to get to the Duomo from Centrale. Add 15-20 minutes for getting down there, buying the tickets, and waiting for the train to arrive. Once you have chosen which color line you want (and you are in the metro station), you follow the color to your platform. The directions with color markers are on the wall, so follow the signs for yellow. Then, make sure you know the name of the end of the line you are choosing, because you want to go to the correct side of the platform so you are going toward to Duomo and not away. Here is a link with the map and maybe a better explanation:
https://mominitaly.com/how-to-use-the-milan-metro/

The metro line between Centrale and Duomo is line 3, yellow.

A few other things: you asked about a train pass. That depends on your use of trains while in Italy. If this is your only trip, you can just buy a ticket for the trip. The time from Venice to Milan is 2.5-3.5 hours depending on which train you catch, so it sounds like you are planning to leave your hotel between 4a-5a? Finally, if you have luggage and were intending to store it at Milan Centrale (they do have a useful luggage storage), you would need a lot more time. There can be a long line to drop the luggage and get your ticket. You didnt mention this, so it might be unnecessary for me to mention, but in case. It sounds like tight timing and an early morning.

whichever train company you choose you will be on the first train of the day (what could go wrong?). To be at Milano C.LE by 8:15 you would be on the Trenitalia FrecciaRossa 9700 which leaves Venezia Santa Lucia at 05:39 and arrives at Milano C.LE at 8:12.

Alternatively you could use the nicer Italo, first of the day, 8970, leaves Venezia SL at 05:57, arriving Milano C.LE at 08:25.

Either way you have to choose - they don't accept each others' tickets, and Italo do not accept railpasses.

If you need to use the Night Vaporetto service to get to the station in Venice you should know it runs much less frequently than the day vaporettos.

Where is your hotel?

A lot of people think that buying a railway pass could be cheaper and easier than point-to-point tickets, but in Italy it is not. Passes are worth only for people doing heavy commuting - and there are also long range commuters, for example a colleague of mine lives in Rome, works in Florence and every day from Monday to Friday he travels 285 miles in two instances. But even with a pass you still have to reserve places on fast trains, so it is not easier at all.

You no longer have to purchase at ticket at the kiosk for metro. Just tap and pay with credit card as you go through the metro gates. If you use the same card for 2 people the second person can’t use same turnstile. Use one next to the first persons turnstile. Same thing with exit from metro. Worked great yesterday as the lines were long for the ticket kiosks and half weren’t working