A couple more comments as I reread your post(s).
More than likely, some more thoughts will appear, from time to time.
<<"So the main goal of this trip is really to discover the Alps an do lots of day-hikes. That is why I would like to rent somewhere relatively central that would allow us to explore. That is why Wengen seems appealing to me with is relative proximity to the lakes, Brienz, Thun, Grindelwald, Jugfraujoch, Mirren, etc. so we can have a mix of nature and a bit of small cities.">>
I like Wengen. So does Kim:
Wengen is a great place to stay for access to scenic locations and hiking. See a few attached images related to Wengen, and Männlichen. (Unfortunately, the Hotel Eiger referred to in some images has been converted to apartments. Super location close to the station, and the restaurant on the ground floor has gotten very good. Fortunately, available to the public.
This is a spectacular easy walk...not really a "hike." Walk toward Kleine Scheidegg, not the reverse.
Many peope like Grindelwald, too. It is amore commercial than Wengen.
Interlaken is an excellent base for train travel to the rest of Switzerland and onwards. There is main-line service to Interlaken. From Interlaken into the Lauterbrunnen Valley, the lines are meter gauge or smaller. However, Interlaken really does not have much scenery of its own.
<<" Mirren, etc. so we can have a mix of nature
and a bit of small cities.">>
Although Mürren is the correct spelling...watch for umlauts....it is pronounced sort of like Mirren. Close. ;-)
By the standards of Canada or the USA, all cities in Switzerland are "small." Zürich has a population of around 400,000 pepole. The ones that have a substantial metropolitan core include Zürich-- quote from Wiki:
<<"Zürich or Zurich (/ˈzjʊərɪk/, Swiss Standard German: Zürich [ˈtsʏrɪç], German Standard German Zürich [ˈtsyːʁɪç], Swiss German: Züri [ˈtsyɾi], French: Zurich [zyʁik], Italian: Zurigo [dzuˈriːɡo], Romansh: Turitg [tuˈritɕ]) is the largest city in Switzerland and the capital of the canton of Zürich. It is located in north-central Switzerland at the northwestern tip of Lake Zürich. The municipality has approximately 400,028 inhabitants, the urban agglomeration 1.315 million, and the Zurich metropolitan area 1.83 million. Zürich is a hub for railways, roads, and air traffic. Both Zürich Airport and railway station are the largest and busiest in the country.">>
Zürich is quite sophisticated. It's attractions tend to be more cultural than scenic. It is an excellent rail hub.
Geneva, Bern, Basel. are the most "city-like." Although most Swiss have French as a compulsory language in their education, and you will find it spoken almost everywhere, if you would like to practice your pure (classical) French accent and compare it with the Swiss version of French, you could consider a side trip to the French speaking regions. Montreux is easily accessible from Interlaken by slow scenic train via Zweisimmen, or faster trains via Bern.
Luzern is not as large as the other well known cities
However, Luzern, due to its location, combination of lake , mountain and countryside terrain, a very nice old city (full of tourists, unfortunately) . it is the usual recommendation for a starting point for first time visitors to Switzerland. It has pretty good central location. In Switzerland, the regions around Switzerland are called "die Zentralschweiz." The region is the heart of the old Federation.