If you haven't already, it would be a good idea for both of you to thoroughly explore the RS website section called Travel Tips: https://www.ricksteves.com/travel-tips
Likewise, the Man in Seat 61's Beginner's Guide to Train Travel in Italy: https://www.seat61.com/train-travel-in-italy.htm
The trains in Italy are great and very reasonably priced. I particularly like going on the fast ones (300 kmh) called Freccia...
If this trip is for this year, prepare for some serious logistical planning challenges for the basics of lodging and transportation. You may be shocked to see the prices and the lack of availability. If you're planning for June 2025, you don't have to be so concerned about that.
As others have mentioned, flying multi-city will save you time. Both Milan and Rome are good options to fly into or back from. I find Google flights to be the best place to search for flights. Make nonstop a priority. If there is a nonstop to Venice, it's a good place to fly into, but not to fly back from because the dang flights leave so early and you have to be at the airport 3 hours ahead (just like Milan and Rome). Google Flights has links to the airlines to make your reservations. They take you straight to the airlines, but you can also go directly to the airlines separately.
As for lodging, Booking.com is a good resource to use to find a place to stay. With such a short time in country, hotels or B&Bs will probably make the most sense. Find some good options and check their websites to make your reservations. That's typically recommended because the hotels sometimes have rooms that aren't listed on Booking.com, sometimes you might get a better price and if something goes wrong, it's easier to fix the problem when you've booked directly.
Finally, there are good forums here that discuss practical travel topics: https://www.ricksteves.com/travel-forum
You'll find them in the drop down box labeled "Select Your Category" under Tips & Trip Reports.