Swiss Rail Pass - and Goldenpass route

None of the trains on the Golden Pass route require reservations, so you can just take whatever train you please on the route if you have a pass or valid ticket.
If you want to make reservations however you do not need the pass (or even a ticket) to do so. On trains reservations and tickets exist completely independent from each other.
So you can anyway safely wait till you are in Switzerland to buy the pass if you wish.

With a second class pass (or ticket) you can always upgrade to 1st class. You then pay the price difference between 2nd and 1st for that sector. You can do that at the ticket office, with the app, from a ticket vending machine and even on board.

The new Golden Pass Express trains have different types of seating than the former Golden Pass Panorama cars. Reservations are mandatory only for Prestige Class, which has rotating seats you can turn so they always face forward. You must have a First Class ticket (or Swiss Pass) to reserve in Prestige Class.

https://www.trains.com/trn/news-reviews/news-wire/swiss-gauge-changing-panoramic-express-debuts-on-golden-pass-route/

For regular First and Second class, reservations are optional, but might be a good idea in high season to ensure you have a seat. I don’t know if that includes late May/ early June, but I suspect it does. The Swiss trains we rode last September, like the trains in Italy and Germany, were fully booked, even in First, so we were glad to have reservations. The one exception was the Treno Gottardo in First, because few people are aware of that new-ish train.

If you do want to reserve seats, you can do that now, without having the Swiss Pass in hand., as WengenK stated. You do have to have it to show the conductor when you actually travel.

Thank you WengenK and Lola!

I was having a hard time understanding that seat reservations and tickets are essentially decoupled -- this is super-helpful information!

Cheers -
John

Remember for the golden pass, all the trains in every class have the same view. Well unless you sit in those seats underneath the driver, but that is not necessary. Watch some vlogs if you're concerned, but the standard advice is just to sit in second class and enjoy.