You do not need the half fare card to purchase half price tickets. You will not be asked for proof you have that card during the purchase. It is on board that this is checked. So you have till the moment you step on board to get the card.
You also do not need to buy tickets in advance. It is only the reservation you need. So you can for example reserve your seats now, and just select “reservation only” in the reductions drop down box when you come to that point in the process.
That gives you time to consider getting either a Half Fare Pass and a ticket, or a Swiss Travel Pass, or a Saver Day Pass for example.