The drive from Salzburg to Bolzano

Where did you look for driving times? It is 4 hours by train. Google maps says 3:30 driving time, which probably means at least 4 hours in reality.

Will you be driving a rental car? If yes, did you rent it in Austria? If yes, are you returning to Austria to drop it off?

If you plan to come back to Austria afterwards, then it is a good plan; the drive is easy (100% motorways), unless you get stuck in one of the frequent traffic jams - the tailbacks can stretch for hours at weekends in July and August!

The Innsbruck-Bolzano section is rather slow by train; but also driving is not as fast as it looks as Tirol imposes a 100 km/h speed limit within its boundaries, and Brenner to Bolzano is limited to 110 km/h.

The poster that wrote about queues is right: the drive is better avoided in late July and August weekends. The worst queues are actually south of Bolzano, but also between Salzburg and Rosenheim you can easily get stuck. You can decide alternate itineraries if you see on Google maps that the traffic is slow: I often drove Salzburg - St. Johann - Wörgl to skip queues (shorter than the highway but slower under ordinary circumstances, but faster if the highway is stuck) - and actually a little favourite of mine is the Bernau - Aschau - Nierderndorf cutoff.

Another handy cutoff if there is a wait at the Salzburg border point (it happens frequently if covid controls are enforced): exit Salzburg from the airport road direction Bad Reichenhall. There is a small border crossing that sees little traffic. As soon as you have crossed the border, turn right and enter the highway direction München - having skipped the wait at the border.

In my opinion driving from Salzburg to Bolzano deserve a slow route, maybe with some stops along the way. Starting in the morning and arriving in the late afternoon.
You can lead southward to Lienz, than enter in Italy at San Candido and than down to Bolzano.
I don't know the Austrian side (except the Loacker factory in Panzendorf :-P ), but in Italy you can stop at San Candido (nice village) or Dobbiaco's lake, you can visit the fortress of Brunico or of Tures. A lot of opportunity to make the drive not only a transfer, but a real journey.

Ricky's suggested route is excellent!
In summer and in good weather, you can drive via the scenic Grossglockner road to Lienz (otherwise via the Felbertauern tunnel), then west towards Italy via San Candido (indeed a beautiful village). Except the Grossglockner road, it is still a reasonably fast route.

I concur that this route takes quite a bit of time to drive along. I just did it. Traffic was bad--Google Maps' driving time was optimistic. This route is a major logistics artery between the two countries, and therefore has a lot of big rigs and service trucks. Stopping is a good idea. Then, you can rest and absorb the scenery.

I also agree about the Brenner route traffic. I've been hours stuck there.

One fun time a car had lost control of a caravan (camper trailer - you know it has been enough years that I just don't remember the US terminology, I hope you know what I mean) southbound and we were there a good long time. Could see what was going on halfway down the slope. I saw nothing remaining of it other than bits blowing around.

That wasn't the only time it stopped for us.

Remember to make a Zipper if you are stopped.