Bernina express

If you want to do the actual Bernina Express, better get seat reservations now before proceeding any further. Depending on when you want to go, they can be in short supply.

The problem with the Milan idea is that none of their airports (especially with Ryanair...) are that convenient for Tirano and this plan looks like it might be tight for time. Whereas Zurich is easy for Chur - but not cheap. Easyjet Manchester-Basel may also be viable, there are usually 2 direct fast trains per hour Basel-Chur. What about a circular trip with easyjet, one way each Basel and Milan and no doubling back across the Bernina?

Nobody can predict fog months in advance. But short days at that time of year will be a factor

There are also cars named “Allegra” attached to the Bernina Express train. Don’t need a seat reservation. The same ride, you even get the announcements from the Express train. Wife and I just hopped on it today in Chur to Pontresina after our Glacier Express train. Didn’t know about this until we met a couple on the Lucerne to Interlaken train. I had my doubts but he was absolutely correct.

As posted above, short days will be a factor and you may want to travel southward from Chur to Tirano; otherwise, the ending portion of the northward journey, from Tirano to Chur, may be after nightfall.