Buying tickets via app vs. Swiss Travel Pass

You can buy everything on the SBB ap.
You still have to buy tickets on certain private cable cars with the STP but most are discounted.
For example I had the STP last summer and took Wengen-Männlichen at a 25% discount. There was a live person to buy tickets
Lauterbrunnen to Murren is covered 100% as well as Stechelburg to Murren (if you stay in Murren this lift runs a lot later than the one Lauterbrunnen to Murren which stops running around 20:00. I found this out the hard way).

I guess it is more convenient to have the STP if you are not certain your mobile device will work all the time and/or you are in a hurry to catch a train that you did not buy the ticket for in advance.

I’m not much help on buying tickets, because I used the Swiss Travel Pass for my Sept trip and have already bought the Pass for my trip next month. I’m one who’s a huge fan of the Pass. So, to me the cost is well worth it. And I think you’ll find that you will end up using the Pass more than you originally planned. So, I bet you will come out ahead on cost.

How much money is the difference that you come up with in your spreadsheet? If it's, say, 50 francs, then I would buy the pass in a heartbeat. 100 francs, maybe not!
Also, how full is your schedule in the spreadsheet? An advantage of the STP is that, for example, if you get a rainy day in the Berner Oberland and would rather visit Bern than travel up Schilthorn, it would be free with the STP.
But if your planned schedule is already full, then you won't have room for such trips "on a whim", so the extra flexibility won't matter.

Good questions! It's definitely closer to the 100 francs range; around 102 to be exact. The fact that Jungfraujoch is more expensive with the STP than with the Half Fare Card is really throwing things off more than I expected. I also liked that the 1st class upgrade on the Saver Day Pass (35 CHF over the 2nd class) seems more affordable than a one-day class upgrade on the STP (52 CHF). I don't remember first class being necessary on my last trip but I thought it'd be nice on our 4-hour train days since we're traveling in peak season this time.

My itinerary isn't super flexible as we're cramming in our Switzerland time before heading to a wedding in Italy so we have a pretty rigid timeframe and want to cover specific areas we didn't see last time.

The downside to my plan seems to be having to manage multiple tickets/passes for different days, not having the flexibility on the non-Day Pass days, and also that I can't guaranteed that I included every single train ride we might take in my calculations.

Andrea: "Can I buy tickets for all of those things on the SBB app?"
Last year, while onboard the train from Appenzell to the Zurich airport, with a free pass, my husband was able to purchase tickets for us to continue traveling from the Zurich airport to downtown, using the SBB App. It was great, because that saved us time from going up a flight of stairs to the ticket booths and then back down again to the platforms.
We already had an account with them because we had purchased the STP, for later in our Switzerland trip.

Hopefully you know that the SBB prices quoted assume you already have the half-fare card; otherwise the cost of the route would be double.
Also, if you show the Pass to receive a discount, it's considered a use day, even if you haven't used an included train route.
WE found we did more activities since they were covered by the STP, like taking boat rides, and/or going to Mt Rigi, Mt Sanserhorn, etc.
We found the extra convenience worth the cost.
Safe travels!