Annika has offered some good ideas, responsive to your thoughts. She has mentioned one of my favorite train rides - "The Lötcshberger" from Bern (or interlaken with change at Spiez) ) to Brig over the old, higher scenic route., thence to Visp and Zermatt.
I do not recommend the scenery inside the new "Lötschberg Basis Tunnel" which is faster, and which the timetable thus will choose unless you force an intermediate station on the higher, old route. Goppenstein is good, or Kandersteg.
As she notes, Interlaken is hardly a village, is not quaint, and serves best as a transport base.
Not merely time of year, but, also , altitude is important or even critical in defining where to travel in Switzerland.Some of your suggested places to visit are hardly quaint villages.
Some places that definitely are villages, or at most, very small towns, include Scuol, Ftan, Guarda, Ardez, Sur-en, Bos Cha, Zuoz and Zernez in the lower Engadine. They are at the opposite end of the valley from St. Moritz, thus near the border with Austria, and Scuol is an hour and 21 minutes from St. Moritz by train.
These links may help:
Check the links on that page.
www.myswissalps.com/ac tivities/scenictrips/t rain
Work with a map. This one shows train lines if you zoom in: