If you only want to stay a few days in the mountains, you don't need to go to expensive Switzerland. There are marvellous glaciers and hiking possiblities not far away from the Venice - Salzburg itinerary.
Check Grossglockner, Zillertal, Sulden, Kaunertal, Oetztal.
Possible intineraries:
Venice - Calalzo - Cortina - Toblach - Lienz - Grossglockner - Zell am See - Salzburg
Venice - Verona - Merano - Timmelsjoch - Soelden/Hochsoelden - Oetztal - Salzburg
Venice - Verona - Merano - (Sulden) - Nauders - (Kaunertal) - Landeck - Salzburg
Venice - Verona - Innsbruck - Jenbach - Zillertal - Salzburg
If it must be Switzerland, be aware that Matterhorn is just at the Western side of Switzerland, whereas both, Venice and Salzburg are some 200 km east of the Eastern border of Switzerland.
St. Moritz/Pontresina would be closer to Austria and could be combined with the Swiss town of St. Gallen (baroque cathedral) and Lake Constance: train Venice - Bergamo - Colico - Tirano - Berninapass - Pontresina - St. Moritz - Albulapass - Chur - St. Gallen - Munich or Innsbruck - Salzburg.
Zermatt is easy to reach from Venice, but the journey from there to Salzburg will be long and expensive (unless you have a eurailpass or so):
Venice dp 16.20 - Zermatt ar 23.44, change at Brig. Stay 2 days there and stop over at Bern (2 hrs from Zermatt, trains every hour) and may be Lucerne on your way to Salzburg. Bern/Lucerne - Salzburg: 4 daily train connections via Zurich - Innsbruck ( 6 1/2 hrs).
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