Paid reservation is mandatory on the Glacier Express

While the service is admittedly excellent, you also have the option of travelling with regional trains on the same route for pretty much the same journey time (it just involves easy connections in small stations), without reservation (ticket can be bought on the day from a ticket machine or on SBB website), a hourly service as everywhere in the Swiss public transport system, hop-on hop-off as much as you like, and contrarily to the Glacier Express... Windows that can open.

Just check on sbb.ch website (use the 'via' function from the parameters) : if time is key, whether Glacier Express or Regional train, I would recommend to head straight to Brig from Zürich (with SBB mainline trains) and skip Zermatt. Then the journey is Brig - Andermatt - Disentis - Reichenau Tamins (it is unnecessary to go to Chur and come back) - St Moritz (map here : https://company.sbb.ch/content/dam/i...bbdownload.pdf). The most scenic sections are probably Andermatt - Disentis (you can also go there from Zürich, with a previous further change at Göschenen) and probably Tiefencastel - Samedan.

If regional trains : the connection at Andermatt is a few minutes but it might be a good place to have lunch, remaining there for 2 hours plus these few minutes :-) Or slightly further away, ditto at the Oberalppass 200m above sea level (2 restaurants close to the station - or if in summer, it is also a good place to enjoy a sandwich in front of the lake). Again all of this can be decided on the day, without any need to change tickets - btw it is always cheaper to buy end to end Zürich to St Moritz with the relevant 'vias').