Because the weather is expected to be better in the south: Could it be worthwhile to plan the journey via Klagenfurt and Venice (instead of Danube and Bavaria)? In this case, the last part of the trip from Bellinzona to Zurich would be by train.
If you just take the route in the Po plain from and Italian lakes from Venice to Bellinzona, maybe yes. It isn't such snowy and temperature a little bit higher, but is still winter, too. Even there could be rainy days, which isn't really better than cold days. I remember a tour in March with 8 °C and heavy rain the Gardasee. In Austria and the Alpine part of Friuli you don't get better conditions, because it's inside the alpine climate, even if go more south like Wörthersee etc. Winter in Slovenia is stronger than in the German Allgäu, generally. It's a little bit changing in Carinthia and the lake area, if looking to spring 2 month later, getting warm earlier than on the north side, but can't expect this in January.
