Generally, at www.sbb.ch/en, scenic trains with panoramic car are indicated by "PE" in a red block. If you want seat reservations for these, you have to buy them from the individual train operators and these are not covered by the Swiss Travel Pass. The Glacier Express, Bernina Express, and Gotthard Panorama Express have mandatory seat reservation fees. Other trains have optional seat reservation fees.
You can purchase seat reservations alone without buying a ticket. Travel ticket could be a full fare ticket, half fare ticket if you have the Half Fare Card (120 CHF for 30 days), a Swiss Travel Pass, or a Saver Day Pass.
https://www.glacierexpress.ch/en/
https://tickets.rhb.ch/en/pages/bernina-express
https://www.sbb.ch/en/leisure-holidays/trains-boats/panorama-trains/gotthard-panorama-express.html
https://www.gpx.swiss/en/
What is the best ticket strategy? It depends on your other travel plans, where you are going, when you are going, what you will be doing. Unfortunately, there is no single strategy that will suit everybody.