The italian timetable is not fully publised yet (and i heard that there are might Track renovation on the Route from Milan to Tirano )

If you wanna go this journey in one day may cancel the stopover in St.Moritz and catch from Tirano to Bernina Exprees direct to Chur or take Regionaltrains with changes in Pontressina and Samedan to Chur (saves you one 1hour)

If there are really track renovations you might catch the Bernina Express Bus from Lugano to Tirano :)

Here you can find information about the Bernina trip and other scenic routes in Switzerland. 

https://www.swissactivities.com/en-ch/panoramic-trains-in-switzerland/

For Milan to Tirano, watch www.trenitalia.com. I'd expect the full timetable there within a few weeks.