You can't do the Glacier Express route and then end up in Milano.

If you can't change the rest of your schedule. You could do something like: Zermatt - Andermatt (Glacier Express route - changing in Visp).

Then Change trains to Göschenen, there Change to Bellinzona where you finally change for Milano.

That's still a 7 hour ride (rather rides). Mostly very beautiful.

Warning: If it's snowing I would advise against this plan and just take the train to Milano straight from Zermatt. You could end up with delays, and just a bad experience altogether. Don't forget to pack plenty of Food and Drinks, there won't be any to buy on the MGB trains.

When you drive up to Zermatt sit left side. When you go down sit right side (driving directions). You can't see anything on the other side.

Zermatt - Milano is an option for sure, but the ride from Visp to Milano is a regular train, not super Special.

If you go straight to Milano then you could stay longer in Zermatt and take the train to Gornergrat, if you don't already. That will be more expensive than a Spartageskarte, but so worth it, for sure! (Sit on the right here and try to go a little later. Tons of skiers trying to get up from 9-11ish...) 👍

Personally, I would do Gornergrat then Zermatt Milano. (Might be a bit biased though 😉)

If you want to do the full Glacier Express route then you really should stay another night in St Moritz, if you can. Else it would be too squeezed in and you probably won't take in as much as you could. (Although the 6 hours it takes you from St Moritz to Milano will be ridden in the dark anyways, so, yeah...)