The only one that would be an advantageous to buy in advance is the Zurich to Munich. That is because it is an EC train and you can get big discounts to purchase it now. These prices rise as the actual departure date approaches, so early birds get the best price. From what I have seen, you will get the best price if you buy from Deutsche Bahn. You can get the discount for a Swiss Half Fare Card by clicking on the discount box and selecting "CH Halbtax Abo (without RAILPLUS)". These are Flexpreis Europa Tickets and can be exchanged for a fee. I see 121.25 EUR for two 2nd class tickets for Sept 7.
www.bahn.com/en
The rest of the trains and cable car can be bought anytime before boarding, either at a manned station, ticket vending machine, or the SBB app. You just select the discount "Half Fare Card". When the ticket inspector comes by, you show your ticket and Half Fare Card. That also applies to the EC train to Munich.
www.sbb.ch/en