This December trip is Munich > Zurich CH via St Margrethen CH. DB says it is an ECE train and a "layover" of 1 minute in St Margrethen. Google shows 0 changes. SBB has the same schedule, but lists both trains as EC, a retro, stylish train that crosses borders. DB uses the term ECE, EuroCity Express.
I read suggestions for Switzerland > Austria. Also, there is Italy > Switzerland. However, that requires a train change at the border.
Buy a Munich > St Margrethen ticket in Germany and at the border present the Saver Day pass to the (new?) conductor?