Sirroujo87 With your Swiss Travel Pass, you could take another route, for example Interlaken - Zurich - Milan or Interlaken - Lucerne - Lugano or Arth-Goldau - Milan. It would take more time but it would be more comfortable with the train. It is not expensive, you pay only from the border station Chiasso to Milan.
I don’t know if they restrict the amount of luggage in the bus. The bus tickets are now available on sbb.ch or trenitalia.com from Domodossola to Milan.