Rail options between Switzerland and Germany

Have you checked the fares for the individual legs and any usage you may have while in Interlaken? Do that and see how it stacks up gains the cost of the Pass.

A pass of some sort is worth considering for the Swiss part of your trip (more on that below), but not for the German part: you do not travel enough in Germany to justify a pass there.

Instead, you should buy tickets in advance for the Zürich-Munich-Passau journey (on Deutsche Bahn, www.bahn.com) - technically you only need to do so for the Zürich-Munich train, but it is easier to buy it all in one go (even though it might cost a few extra euros). For the Passau-Munich leg, you can probably use a Bayern Ticket as the cheapest option (it is a regional day ticket valid on any regional trains for the day after 9AM, quite a good deal at 34€ for 2 people travelling together).

As for Switzerland, you have 3 options:

• Swiss Travel Pass for 6 days: least hassle since intercity journeys within Switzerland would be fully included (without even needing to buy tickets), but once you get to the Berner Oberland, you "only" get 25-50% discounts on the mountain railways (Jungfraujoch, Schilthorn, etc.)
• Half-fare card: 120 francs card that gets you 50% off every train ride or mountain railway. Usually a bit cheaper than the Swiss Travel Pass, but a bit more hassle since you need to buy tickets.
• Berner Oberland Pass: never tried it, but it could work out since you are only staying in that region. Full fare tickets from Zürich to Luzern or Bern (limit of the area of validity of the pass) aren't that expensive.

+1 pass in Switzerland, tickets in Germany.

There are many threads on sorting out a Swiss pass when you are in the Berner Oberland. For sure worth parsing out the several options. Google Top of Europe Pass.

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If you are just spending time around the Bernese Oberland then the "Berner Oberland Regional Pass" is probably the best option.

https://www.regionalpass-berneroberland.ch

For the ZRH - Interlaken part you can buy a "Saver Day Pass". This is a day pass, allows you unlimited usage of the Swiss Public Transport system, and is relatively cheap when bought in advance. Goes on sale 2 months in advance. Because it is fully flexible you do not need to worry about schedule changes.

https://www.sbb.ch/en/travelcards-and-tickets/tickets-for-switzerland/day-pass/saver-day-pass.html

For Interlaken to Passau just book a ticket in advance on www.bahn.de. You can do the whole trip on one ticket.

Note that there are more attractive places to stay at than Interlaken if you want to spend time in the Bernese Oberland.

as noted, use a site like Rome2rio and price out individual legs. I think Google now does this too. And if you haven't already, make the Man in Seat 61 your best friend.

I would not use rome2rio or google to price out individual legs. rome2rio almost consistently shows longer train times and higher ticket prices than is really the case.
And be aware that in this part of Europe to get from one place to another by train you only ever need one ticket.

The best place to look for prices and times is the national railway of the country where the trip starts. So in this case you would look on www.sbb.ch

Yeah Rome2Rio is a mess. Bahn.com is easy and reliable for all train trips in Europe, though sometimes it's better to go afterwards to the individual county's website to confirm prices, platforms etc.

The Swiss rail website in the post above is also very good.