I just wrote a very long response and lost it somehow. I do not have the patience to reconstruct the whole thing, but will provide an abbreviated version and can fill in more details if you are interested.
Starting with the end of your “Swiss time” and working backwards, my suggestion is that you consider one of the Gotthard Pass scenic routes between Luzern and Lugano or Locarno, instead of the Bernina Express.
The Gotthard Panorama Express is a once-a-day boat plus train combo from Luzern to Lugano:
https://www.sbb.ch/en/leisure-holidays/trains-trips/rail-travel-specialtrains/panorama-trips/gotthard-panorama-express.html
The Treno Gottardo is an all-train route from Luzern to Locarno on Lago Maggiore:
https://www.mystsnet.com/en/experiences/scenic-routes/treno-gottardo/
These two trains ride the same rails enjoy and the same views of snow-capped peaks, hanging glaciers, and steep valleys. The tracks use switchbacks and spiral tunnels to ascend to and descend from the Gotthard Pass. You can see these on the rail map linked below.
Locarno will offer an opportunity to visit the famed Val Verzasca. You would need 2 nights in Locarno to do this, so may not have time unless you limit your time elsewhere.
https://www.ascona-locarno.com/en/explore/valle-verzasca
From Locarno, you also have the opportunity to go to Malpensa airport without passing through Milan: take the ferry down Lago Maggiore to Stresa, overnight there, and use the scheduled bus service directly to Malpensa airport terminals (about 1 hour away).
The Luzern area offers a number of hotels in the luxury category, such as this spa hotel on Rigi (a nearby mountain, accessed by boat to Weggis and cablecar up from there):
https://www.hotelrigikaltbad.ch/
https://en.hotelrigikaltbad.ch/wellness-spa
You can expore other activities, such as hiking, on Rigi on their website:
https://www.rigi.ch/en
My suggestion is that from Turin, you enter Switzerland at Brig, 3 hours by train from Turin, with changes at Milan or Novara, and maybe Domodossola. From Brig, make your way toward Luzern with an overnight on the way. Use this Swiss rail map to see your options (as well as the scenic routes from Luzern to Lugano and Locarno suggested above):
https://cdn-doc.myswissalps.com/docs/default-source/rail-network-maps/swisstravelpass.pdf?sfvrsn=7788603a_56
The map includes all of Lago Maggiore and Malpensa airport as well. It also shows just how far east you would have to travel to take the Bernina Express, and then how far back west you need to go to get back to Milan (just off the map below the towns of Chiasso and Como) to reach the airport.