What train pass should I get?

How long is your trip? How many days are you in each location? Likely the Swiss Travel Pass or the half fare card.

2.5 days in Bern
2 days in Lugano or Locarno (still deciding which one)
3 days in Zermatt
2 days in Wengen
These are full days in each location with a travel day in between each destination.

Assume your 2.5 days in Bern means you are traveling to there in the morning after arriving at Zurich airport. Where are you going after Wengen? Back to Zurich airport?

A 15 days Swiss Pass is $547. A half fare card is 120 CHF. If you download the SBB app you can look at the costs of the fares between locations. Divide them in half for the half fare card. That will help you decide which works best. Are you planning to go any museums? Many are covered by the Swiss pass as well transportation locally such as the BO area around Wengen. We traveled for two plus weeks using the half fare card. It was slightly less than the Swiss Pass but we had to buy tickets for each train/bus/etc.

I just did a bunch of number crunching between the Travel Flex pass and Half Fare pass for our trip to Switzerland. Our trip is longer than yours but similar in that it involves some long journeys (either SBB trains or the scenic trains) and some time using lifts in the Berner Oberland.

In the end, the pass options came out costing about the same once money up-front vs savings were compared. I'm tending toward the Half Fare pass because the cost is way less up front with expenses being a little higher along the way, and this gives us more flexibility. For example, if the weather is not good we just won't go on an expensive lift like the Jungfraujoch that day - and save our money for a short hike and nice dinner or something else that's enjoyable.

I notice that some lifts in the Berner Oberland (the Eiger Express, Jungfraujoch and Kleine Scheidegg) are actually cheaper with the Half Fare pass. They give a 50% discount with the Half Fare and only 25% with the Travel and Travel Flex passes. All the other lift costs are exactly the same discount - 50% with either pass.

When we leave Wengen, we will be going to Zurich for our departure flight the next day.

I think you will find the 15-day Swiss Travel Pass will work best for you.

Thank you so much for the helpful information everyone! I definitely feel a little less overwhelmed on this detail of our trip. I appreciate everyone taking the time to offer us some guidance. Happy travels!