Suggestions for weather in Switzerland first two weeks in August.

Not to be glib, but pack for both heat and cold. Plan for the possibility of 80s and 40s. I do that every year. Only my tops vary: a couple of short-sleeved shirts/blouses/tees, a couple of long-sleeved shirts, a cardigan, a mid-weight fleece, a light packable “puffy” jacket and a windbreaker. A rain hat is useful, too. I bring three pants including jeans. As we hike a lot, I always included wool tees of both short and long sleeves. They are great for anything up under 70 degrees and can be worn several times before washing.

While I doubt you'll find anywhere in Switzerland that is at sea level, you are correct that in general the valleys are warmer than the higher elevations. And some areas will have more rain than others. You can Google for historic weather averages in each of the cities and towns you will visit. Best to bring layers and have a plan for rain.

Just follow what the above posters have said- 80's in the cities and 40's in the Alps, ( mornings and evenings.) Enjoy this beautiful country.
Safe travels!