ticket for Glacier Express

For two trips, the Half-fare card does not make sense, unless you want to travel 1st class (unnecessary).

For the Glacier Express, now that you have your reservation, the cheapest option is a Saver Day Pass. Buy it as soon as possible (max 60 days pre departure): prices vary, but they are always less than the price a regular ticket from St Moritz to Zermatt. Make sure to buy a 1st class day pass if you have a first class reservation.

For Zermatt to Basel, Saver Day Pass is likely to be cheaper too, but compare with the price of a "Supersaver" ticket which is specific to a given departure. No reservations needed.

The easiest way to purchase is the SBB app, in my experience. Make sure to set up a profile without a half fare card, because the SBB system assumes that you have one by default!
The app will automatically suggest you the Saver Day Pass, saving you the need to compare.

Otherwise the SBB website is fine, with the same warning.

EDIT: I read the post a bit quickly and I see now that you have a day in Zermatt in between. Give the high cost of mountain rides there, a half fare card could be worth it!
I also see that you start Glacier Express from Chur rather than St Moritz, but if you do not get the Half-fare card, a saver day pass will still be cheaper than a normal ticket for that journey.

Some other pieces of info to make a decision on what kind of ticket or pass to purchase.
How are you getting to Chur to begin with? By train? From where?
While in Zermatt are you planning on further excursions up the mountains by rail or cable car? This can depend on weather.

The 30-day Half Fare Card costs 120 CHF and is really a function of the full fare cost of the tickets for all the journeys you plan to do. If the full fare cost is more than 240 CHF, the HFC will save money, even if you are in Switzerland only 4 days. The HFC gives 50% off for all transport tickets, including city buses and trams, mountain cable cars and railways like the Gornergratbahn in Zermatt. You can also use it for 50% 1st class tickets, so it works for buying a 1st class ticket on the Glacier Express then 2nd class for everything else.

You can get the full fare prices at www.sbb.ch/en by clicking through on each journey with your name, date of birth, and checking "No Discount" then also checking "Point to point ticket". Example: Chur to Zermatt 1st class point to point no discount is 210 CHF. You almost cracked the nut on the HFC. Riding the Gornergratbahn to the top is 88 CHF each way full fare.

Sometimes, deciding the best ticket strategy for Switzerland is like an MBA homework assignment. The Swiss Travel Pass is often the most expensive option, but people like it because of the convenience. No tickets to buy beforehand, just get on the train. Except it only works for travel to inhabited villages. It only gives discounts on mountain railways like the Gornegratbahn. The Saver Day Pass option can sometimes be the best strategy, and sometimes the Supersaver train-specific option works if you can live with a set itinerary.

Good luck in your math exercise.