Have you got any particular places of interest you want to see on route?
Thinking about it, 2 to 3 days won’t give you much visiting time. We’ve gone down through Germany in the past, and enjoyed the Romantic Road. Otherwise I think Google / Michelin maps for the quickest routes.
If taking a week or more, then that would bring a whole host of possibilities.
I was going to advise the same: Go down through Germany( no tolls), Austria has a toll( you can buy a Vignette sticker for 10 days or 1 month - 12 is euro for the 10 day) or you can get a digital one at a machine at the rest stop / garage, the Brenner Pass fee is payable here too.
Asinfag is the service company and you can do the Austrian and Brenner tolls online.
As you are under 3500, you don't need the toll payment thingy (tech term) for Italy, it can make life easier but a pain to get....I just pay as I go (take euros).
Austrian border to lake Garda usually costs around 40 euros in toll fees ...
The Italian Autostrada tolls are usually when you get on and off.
In Summary: Austria toll then Brenner Pass toll then Italy Autostrada toll...