The SBB app is invaluable to assist in planning all forms of transport including trains, trams, buses, mountain railways, boats, cable cars etc.
A Swiss Pass is definitely worthwhile, even though it is not cheap. Alternatively you can get a half fare pass for month and get saver day passes and tickets as you need.
Not sure how long you’re planning - the other thing worth considering is just base yourself in a central place (such as Zurich or Bern) for most of the time there - take day trips or even an overnight as required. With a pass and a base you can be flexible and go where the weather is best on a given day. (In summer the weather can be less problematic but even the you never know). Nothing worse than having booked that once in a lifetime trip to Matterhorn, Jungfraujoch or somewhere else and spending the time in clouds looking at clouds !
I know people rave over the Glacier and Bernina express journeys. I’ve never done either, but have completed both on local trains which were amazing as well. The pros of the two “Express” trains are the panorama windows and commentary. Though after weighing up the pros and cons we went for local trains (much quieter) and allowed for some interesting enroute stops and detours - more like a hop on hop off train along the same route!
But don’t bother with car, trains run on time or very close to on time 98% of the time and are a really relaxing way to move around unless you want to visit some very specific places that are difficult to reach by PT .