For me the Berner Oberland is the PNW of Switzerland - you should feel right at home! ;-)
whenever we’d hike, the number of people would dramatically decrease the longer we went. After an hour or so, we’d often have the trails to ourselves. I doubt we’ll be so lucky there in the Berner Oberland!
Actually you will find that here too, even in the most tourist-dense areas. The shortest, easiest hikes are the most popular and as the difficulty increases the number of people on the trail decreases.
We might take one day to visit Spiez; do you recommend it?
Spiez is okay - the castle is quite interesting and the waterfront is pretty, but I like Thun better (only another 8 min further by train). It is a larger city with a lovely old town area and also has a castle. You can climb up into the four towers for great photo opportunities.
https://schlossthun.ch/en/
You can walk through the old town area, along the river, over wooden bridges, in the Bälliz shopping street with outdoor cafes (highly recommend Steinmann a local, family run business). Some photos of the city can be found here
:
https://www.interlaken.ch/en/experiences/poi/thun-old-town
Schadau park is a nice location to see the lake and mountains. Thun is at the end of the lake and you can see more of the mountain panorama from there than from Spiez. The photos on this link are taken with a bit more snow on the mountains, so not exactly what it looks like in summer.
https://www.schloss-schadau.ch/en/park-and-surroundings/english-garden