Thank you both for your helpful responses. I discovered that I can make reservations for train trips, or upgrade from second to first class if desired, on the SBB mobile app even without the Swiss Travel Pass uploaded into the app. This of course means I will need to show physical ticket, app and ID, but at least I was able to purchase the additional aspects of each part of the route in the app.
I was impressed with the app and its abilities to allow all this, provide comprehensive info about trains and transfers, allow smooth purchases, send timings to calendar - a very neat app.
Apart from the fact one can’t upload a previously purchased Travel Pass! This surely could be done on the basis of Pass number and other information. Other train companies (Eurostar, other UK companies) allow and enable this - a helpful facility.