Lake Como to Switzerland Transportation Question

Has anyone ever taken a day trip to Switzerland while visiting Lake Como via the Bernina Express?

I'm looking at taking the train from Varenna to Tirano then hopping on the BE to St. Moritz. I know it will make for a longer day, but the train ride itself is the main purpose of the day trip because it's said to be the most beautiful and scenic in the world. But I'm wondering a few things...

Is it worth taking the BE all the way to St. Moritz or is there a stop before St. Moritz that gives you a better taste of Switzerland, especially considering we will be there in September/October and St. Moritz is a ski town.

If not a better stop before, does anyone recommend any easily reachable destinations from St. Moritz like Sils Maria to spend a few hours walking around and grabbing a late lunch?

Lastly, if anyone has done the trip would they recommend taking the train to Switzerland and then on the return, or would we be better off taking the BE train to St. Mortiz then taking one of the postal buses back. I know there's one that runs via Chiavenna to Menaggio, which is likely where we are staying on Lake Como.


TIA.