Como or Maggiore

Either lake would be a good choice, and I suppose which one to choose would depend on what kind of "experience" you're looking for? I've been to both, but tend to lean towards Lago di Como and the town of Varenna in the mid-lake area. From there you can take ferries to Bellagio, Mennagio or other points on the lake. There are some very posh old Villas in that area. The train trip from Milano Centrale to Varenna-Esino is about 1H:10M as I recall.

If you decide to visit Lago Maggiore, start in the town of Stresa. From there you can take tour the ornate Borromean Islands, including Isola Bella - https://www.youtube.com/watch?v=jrQrkLWvqWY&t=101s .

One other thought occurred to me. You might consider taking the train from Milano Centrale to Locarno, CH, which is at the other end of Lago Maggiore (take your Passports as you'll be in a different country). The trip is about 1H:55M using a direct train. It's a beautiful city (IMO), and while there you could take the cable car up the mountain to Cardada & Cimetta (fantastic views!!!) - https://www.cardada.ch/en/cardada-cimetta/the-place-to-be .

Thank you for this thorough answer!