Swiss pass and seat reservations

You absolutely don't need seat reservations on these trains. The majority of Swiss trains are mass transit, with the exception of the few tourist scenic trains (Bernina, Glacier, Golden). People are constantly getting on/off and there is no "risk" of not having a reservation. I'd actually find it more annoying to have one, as you'd likely be kicking someone out of your seat when there are open seats around you. Some people do get a reservation on the Interlaken to Luzern portion, which you can do since it's technically part of a scenic train line, however I've ridden it without a reservation with no problem.

If you want to see how busy a train will be, go on the app, look at the dates/times you're interested in and you'll see next to the Platform number a "1" with 3 people icons and a "2" with 3 people icons. This represents how busy they anticipate 1st and 2nd class to be. If there is 1 or 2 people filled in with black, it won't be very busy. If it has 3 people filled in with red, it's typically a very busy train and you may find seats a bit harder to find. Your options are to take a less busy time or just know that when you get on that particular train you may search a bit longer for a seat, wait for one to open up at the next stop (it always happens) or just stand for a bit. This typically only happens during rush hour times or on trains to/from the airport, etc.

You'll also find that 1st class is much less full than 2nd, so if you need to take a particularly busy train time somewhere I'd personally rather upgrade to 1st class on that section than pay for a seat reservation.

We have about 18 days total past two years with first class Swiss Pass. Never a seat reservation (other than Glacier Express) and literally never any issue with seats. I would not worry about this. Trains rarely crowded at all inour experience.
This included ski season and mid August.

Thanks for your quick responses! We leave today from US to Amsterdam. This was helpful. Our passes are 2nd class. I like the tip on getting a 1st class upgrade instead of a seat. I kind of regret now not getting the 1st class swiss pass. Does anyone know if I can change them?

we do not plan to take the 5 hour panoramic. I'm looking at the one that is about 3 hours through Airolo so our passes will work.

Your pass would also work on the fast way with the Gotthard Base Tunnel, but the route via Airolo is more scenic, although you need to change trains in Bellinzona.

Coming from Buchs, will you already be on the Railjet from Austria? If you have seat reservations on that, they will continue on to Zurich, where you change to a train to Bern.

Only Swiss IC trains have seat reservations for sale. The exception is the Luzern-Interlaken Express, which has seat reservations sold by the train operator, Zentralbahn. They cost more than SBB seat reservations. If you do decide on these, make them for the right hand side of the train for the best views.

Looks as if you can upgrade each trip on The SBB app as you go, but cannot exchange 2nd class for first class pass. Probby not worth the trouble imo.