I think your plan for 2 nights and a full day in Milan is a good one. I enjoyed being there after an RS Village Italy tour in 2017 so much, that if anything, I'd add a night so that you'd have 2 full days.
I can highly recommend the Hotel Berna (https://hotelberna.com/en/). It has lovely rooms. Mine even had a mini-fridge, a rare amenity anywhere in Europe. The breakfast buffet was great and they had a good room service menu that was from a restaurant nearby. It's an easy walk from and to Milano Centrale where you will arrive from Malpensa and where you'll leave from to join the tour in Varenna.
Milano Centrale is also where you can take the metro to other destinations in town. Click on the Where We Are link at the Berna's website for a little more information.
There are many things to do and see in Milan. I made a point of going to the cathedral, Duomo di Milano (https://www.duomomilano.it/en/). I got a ticket that included it, the Archeological Area ruins below and for the elevator to the roof. It doesn't go all the way up, so there are a few steps to the main level and more all the way to the top. I stayed on the main level with the buttresses and the gargoyles. The views were great, but my favorite part was being up close to the architecture.
The Galleria Vittorio Emanuele II (https://www.yesmilano.it/en/see-and-do/venues/galleria-vittorio-emanuele-ii) and the Duomo are next to each other on the Piazza del Duomo. The metro arrives right there.
I wish Stanley Tucci had done his series before I was there. I would've definitely gone to the Osteria del Treno (https://www.osteriadeltreno.it/?lang=en) if he had. I'd go back to Milan just to have a meal there.
You are very smart to start your planning for the fall of 2024 now, especially if this is your first trip to Italy or Europe. You have lots of time to research, and there are some good resources right here to do that.
Travel Tips for all aspects of European travel: https://www.ricksteves.com/travel-tips
Having found the forum for Rick Steves Tours, you probably already noticed that there are many forums related to the Travel Tips under the Travel Forum Tips & Trip Reports menus.
Explore Europe for Italy: https://www.ricksteves.com/europe/italy
Travel Forum for Italy: https://community.ricksteves.com/travel-forum/italy