Bernina Express route

Noting the Glacier Express doesnt go to Zurich. It's in the direction of Chur to Brig/Zermatt. If you were starting in Zurich, you'd have to go to Chur to connect to the GE.

I have booked seat reservations in September for the Bernina Express, Tirano to Chur route. I am not expecting to stop in St. Moritz but thought the route would go by St. Moritz.

The Bernina Express bypasses St. Moritz. It runs from Tirano via Pontresina, Samedan and Thusis directly to Chur. But you could, of course, take one of the normal trains that go directly from Tirano to St. Moritz and then from there take another train back to Samedan and on to Chur. However, I would say that the detour is not worth it in terms of the scenery. The most beautiful sections are those over the Bernina and Albula Passes, and you'll also travel over them on the direct route.

you could look for one of the St Moritz train driver eye youtubes to see what you might miss - I think you may conclude it isn't worth it = I went and that was my impression

If you look at a rail map, there is a little triangle between Samedan, Pontresina, and St Moritz. The direct Tirano to Chur takes the short leg of the triangle from Pontresina to Samedan. Stopping at St Moritz would be a needless "long-cut". There is really no significant scenery in the 4 miles between Samedan and St Moritz that you won't see going from Pontresina to Samedan.

the youtube https://youtu.be/rbW4-x6vJmk?si=UckZ8br9gw7-LGcV from minute 1:00 to minute 10:30 gives an idea of what's between St Moritz and Samedan.

It is a little snowy so you have to imagine the green forest.

After being on both the Glacier Express and the Bernina Express route— my advice is to skip St. Moritz.
It is famous— but not scenic.