Best place for a Base.

We used Grindelwald as a base and it worked very well. Plus it was so nice to return there after touring.

https://community.ricksteves.com/travel-forum/trip-reports/lucerne-62eb4d28-60ab-4837-b4b4-91a2486eb732

Perhaps my Trip Report from 2021 will offer some helpful hints.
I'd suggest staying in Pontressina instead of glitzy St Moritz. Also, I'm unsure if you're actually staying in the Berner Oberland.
If you're departing flight isn't too early, stay in Lucerne the night before.
You're in for a treat in this beautiful country! Safe travels!

"I'd suggest staying in Pontressina instead of glitzy St Moritz. Also, I'm unsure if you're actually staying in the Berner Oberland."

I considered Pontressina but with having an early morning train in St Moritz, I thought it was best to stay there. As for staying in BO, we had not got to that stage of the planning.

So, you’re trying to decide where to sleep from Sep 11 through Sep 17. What time does your flight leave on Sep 17?
The Jungfrau deserves five nights and I would sleep in the Lauterbrunnen Valley. That leaves one night and if you have an early morning departure sleep near the airport your last night in CH.

My flight leaves 1:40 pm so plenty of time to get there that morning. I will look into the are you suggest.

for a 1:40 pm flight you need to be at the airport 3 hours prior which is 10:40.

If you are in Lauterbrunnen the trains take 2H40 into the airport. So the latest you could go is on the 7:32 am train down the hill from Lauterbrunnen to Interlaken Ost, change to a main line train to Bern, change there for a train right to Zurich Airport, arriving 10:14.

The only problem is that the connection at Interlaken Ost is only 7 minutes which with luggage and the stress of the last chance could be tight. The change in Bern is 10 minutes, again you will need to get a wriggle on. At Bern the two ways down and up between the tunnel and platform are a staircase from one side and a ramp on the other side. The ramp is quite long but relatively shallow, the stairs are quite steep. I prefer ramps with my disabled wife and we usually make connections easily, but we know exactly where we are going - many years experience there - and we don't have a plane to catch.

You only need one thing to delay you and the plans can go out the window, and unexpected things do happen sometimes.

If I were in your shoes I would have the two of you on the 7:02 (also includes a 3 minute cross platform change at Spiez as well as the ones in Bern and Interlaken Ost), or the 6:32 which only has one connection at Interlaken Ost and the train goes then straight to the airport.

But you know the two of you, your tolerance of risk and acceptance of additional cost, and your overall fitness.... Just don't start for the airport to late. Grab that last pastry and put it in your pocket instead of an extra cup of coffee.....

We used Mürren as our base for the BO and went up both the Jungfrau and the Schilthorn. You could spend your last night in Lucerne and have a shorter trip to the Zurich airport.

At Bern the two ways down and up between the tunnel and platform are a staircase from one side and a ramp on the other side.

There is also the option of taking the elevator or escalator (around the E or F end of the platform) and using the walkway above the train tracks to change platforms, rather than walking down through the tunnel.

I agree about that overbridge and thought about including it - but it is inconvenient to much of the train usually and not, to me, particularly conveniently signed. Following the crowds will probably lead to the stairs, and then the ramp.

Unless going out to the road I find using the overbridge to change platforms not too great.

We have stayed in Wengen on our last trips to BO and will be doing so again in July. I think it’s all personal preference. We prefer being up in the mountains and the charm of a car free village. Some prefer the valley of Lauterbrunnen. Some prefer the quaintness of Murren (though I personally found it a bit too small). Grindelwald is a great base to the mountains as well. And I agree with the previous posters about Lucerne. I think a day there is not to be missed. Perhaps travel there the morning of 9/16, enjoy the day, and head straight to the airport from there on the 17th.