A couple of comments to add to Arno's thoughtful summary -
Parking in Lauterbrunnen main garage is cheap. I recall about 6 CHF per day...certainly nowhere near the 30 or 4o CHF per day(or more) at "big" city train stations.
To illustrate Arno's point, even at S'charl, way, way back in the valley quite a drive in from Scuol, at the "back door" to the Swiss national Park, the parking lot has a central parking clock. Pay money and leave a ticket on your dashboard....
However, you're less likely to have to pay to park in Bulle....(best guess ;-) )
When I rent a car in Switzerland, I usually rent a small sedan or small station wagon. Many do not have enough luggage space for two people traveling with routine clothing plus hiking boots, and suitable clothing. Some bags go on the back seat. If I plan to have 4 people plus luggage, I end up paying significantly more.. Check the rates and luggage capacity carefully, and remember that the stated capacity can be optimistic in estimating the size of a "large" suitcase.
Self-service laundry is rare in Switzerland compared to many other countries.
I've visited Sonogno a couple of times and think a car is very useful. There is a parking lot.
PS -High on my list for the area is a striking architectural destination, in the Val Maggia:
It may be disguised, but Avis, at least, has had a one way rental charge buried in the rates. The few times that I have used it, it added 75-100 CHF to the rental. They did not call it a one way charge; I found it by comparing rates to various return points.
It is true that car travel can be faster than train travel for many destinations...unless your destination is a city. The last few km plus parking can be very slow. On well traveled routes, there can be several trains per hour. Out in the countryside, they have become less frequent as more and more Swiss use their cars.
Interurban between major cities now usually runs every 30 minutes, however.
Finally, if I end up paying a reasonable amount more to enjoy the train rides, I consider it money well spent - especially when I cross the Alps, as you will. Many times the trains go places where you cannot get ex ept on foot....no roads.