Swiss travel pass vs Half fare pass

The longest period you can buy a Swiss Travel Pass is 15 days. You have the option of 15 days consecutive (CHF 513, 2nd class), or 15 days spread over a maximum of 30 days ("Flexi" pass, CHF 563, 2nd class).

Details are here: https://www.sbb.ch/en/leisure-holidays/travel-in-switzerland/international-guests/swiss-travel-pass.html

I don't think there is anything to stop you buying a 15 day consecutive pass, followed by an 8 day one (15+8=23 days, CHF 513 + 418 = CHF 931). Unless you are doing a lot of long distance travel, which I doubt as you are spending whole weeks in places, an Half Price Card is probably the most economic option (CHF 120 for 30 days, plus all your fares at ½ price).
Details here: https://www.sbb.ch/en/leisure-holidays/travel-in-switzerland/international-guests/swiss-halffare-card.html

Also investigate the Berner Oberland pass for your time in that area. Covers Mürren, Kandersteg and area, with slightly better mountain train coverage than a Swiss Pass. 10 days for CHF 390.
Details: https://www.regionalpass-berneroberland.ch/en/

Only way to be sure is run the numbers for the specific trains, lifts, boats and buses you plan to take (plus any activities covered by the pass that you will want to do): full cost for separate tickets (note that SBB fares assume you have a half-price card, so you need to double the cost shown); half-price ticket costs plus the CHF 120 for a monthly half-fare card; cost of travel pass(es) for the 23 days.

The 15-day Flexinpass comes to just under 38 CHF per day of use. That is a deal, considering that some of your days' excursions could be 50 CHF or more. I would get that and use it for the 15 most expensive travel days, and just pay for the rest. Some of your days will be just riding one lift or a bus, and those are not expensive.

We did a multi-location Swiss trip in 2018 staying a week each in Pontresina, Lauterbrunnen, and Lausanne. My analysis led me to buy a Swiss Flex Pass for 15 days, a Half Fare Card, plus a Berner-Oberland Regional Pass (which we buy every year when we go there). The BOReg Pass is terrific for your time in Mürren and Kandersteg as almost all lifts, trains, and buses are included. The Swiss Flex pass only covers most lifts at 50% off, although Rigi is included. The HFC decreased the cost of a BO Regional Pass as well as was useful on some other routes when it did not make sense to use a day of the Flex Pass.

As others have said, the only way to be certain is to run a spreadsheet with your plans. And note that what was completely covered can change from year-to-year. In 2018 the Flex Pass covered the whole trip to the Schilthorn. I don’t think it did in 2019.

Actually the Schilthorn was fully covered on the Swiss Travel Pass in 2019, but for 2020 is 50% covered above Mürren.

https://schilthorn.ch/cmsfiles/Tariffs_Schilthorn_2020_English_1.pdf

This is confirmed on the current validity map:

https://www.sbb.ch/content/dam/sbb/de/infotexte/uebersichtskarte-sts.pdf.sbbdownload.pdf

But Rigi is still fully covered.

Here are some examples of excursions you might be taking that are below that 38 CHF threshold, somgood candidates for just paying cash:

Gondola up to Oeschinensee from Kandersteg, 28 CHF RT ( less if you want to walk back down to town)

Funicular up Allmendhubel from Mürren, 14 CHF RT

Postbus into the beautiful Gasterntal from Kandersteg: price not available as it does not run until summer, but it should be in the single digits each way. I highly recommend this. We have done it by riding in and walking back, and by walking both ways. Or one can rent bikes at the train station and ride the route. The Road in the valleybis restricted to local,residents and the Postbus, so it makes for a nice traffic-free ride.