Struggling with 10-14 day itinerary for two over 70yr olds in Sept coming from US
Can fly in/out of Zurich or Milan
Priorities: Luzern,Wegen/Muren , Bernina Express (seems to be nicest train route)
Possibly Zermatt
Keep trying various itineraries w/o success

Zurich, Lucerne, Grindelwald, Wengen/Murren, Pontresina.

If you start in Zurich and then plan to head over to pick up the Bernina Express, you might want to consider stopping overnight in Bad Ragaz. Every three years they do a terrific outdoor sculpture exhibition: Bad Ragartz. https://badragartz.ch/en

We've been to two of them (the exhibition is free, in case you're wondering) and really enjoyed them. Bad Ragaz is an attractive small town with a truly grand 5-star spa hotel (if you want to splurge) or a quite nice and moderately priced 4-star, the Sorell Hotel Tamina: https://sorellhotels.com/en/tamina/badragaz

One possible alternative to the Bernina Express train route is the Palm Express bus which goes from St. Moritz to Lugano, with a stop in Chiavenna. It's also a beautiful ride, going over the Maloja Pass.

It's much easier to visit these places from Zurich airport than from Milan Malpensa airport. But both is possible.
My proposal: go directly from Zurich airport to Lucerne (1 hr train ride) and relax there after your overnight flight.
After the visit of Lucerne area, go by historic paddle steam boat to Fluelen, by train to Goeschenen - Andermattt and by Glacier Express to St. Moritz/Pontresina/Sils Maria.
After a few days there, go by Bernina Express to Tirano and by Bernina Express bus to Lugano. Stay at Lugano or Locarno.
Take then the scenic Centovalli train to Domodossola and a connecting Postbus over the Simplon Pass to Brig and by small train to Zermatt.
Stay at Zermatt.
Go from Zermatt to Visp and by direct train to Zurich airport.
You may goi from Brig to Interlaken - Wengen etc.instead of Zermatt if you prefer.