I live near to Interlaken and can say that it's been a fantastic summer.
We had no thunderstorms or high winds that we often get late in the day.
And we still got good weather.
I would say that June, September and October are the best times to visit.
Yes it can be cold anything from 3 to 33 degrees with snow or beautiful sunshine but that is the Alps.
Mike
You live in a lovely part of the world - I like your area!
I'm going to visit it again next year (avoiding the toll roads).