Just have a list of ideal weather priorities and rain back ups. For example, the North Face hike from Murren and Schilthorn are best to do in good weather, while Trummelbach Falls is something you can do in rain. Having at least 2 full days gives you a better chance of having 1 good day for your priority activity.
Weather in the Alps is all over the place. It would just be a bummer to go somewhere so beautiful and not get a chance to see the view because the one full day you’re there happens to be clouds and rain.
We spend our summers living in Murren, so I can pretty confidently say that if you kept just one day in Murren, on that day you’re in Zurich you’d be wishing you were back in Murren. Our first trip to Murren back in 2010 was 5 Nights and we quickly realized it was nowhere near enough.
Bern is a fine stopover and does make sense after a long train ride, but in some ways it can be better to just “rip the bandaid off” and get where you’re going. Less checking in/out of places gains you a lot of time in enjoying where you’re at. Being able to wake up the next morning and just enjoy instead of checking in/out again gains you a whole day.
If it were me I’d train from Salzburg all the way to Murren and do 4 nights in Murren and 3 nights in Luzern. Take a train around 6:30 am your last morning and you’ll easily be to the airport by 7:30 and you will have had that whole previous day to enjoy as vacation instead of logistics of moving once again.