Venice from Munich

Many people do it. There is a highway over Brenner Pass. However, there are about 5 EuroCity (EC) day trains that go from Munich to Italy each day. One of them leaves Munich just before noon and goes directly to Venice in 6½ hours. With advance purchase the fare is about 50€. You won't need a car in Venice. Why drive?

You can do it in one day. Assuming you aren't going in winter/early spring when snow in the mountains can cause problems (ask me how I know 😣), the roads are pretty good.

But we didn't go to Venice. We drove straight into Tuscany. Any time we went to Venice we just took the train. A car is useless in Venice.

I agree about taking the train down to Innsbruck, through the Brenner Pass and on into Venice. I haven't looked, but some of the trains might even require a transfer of trains at Verona. There may be another train that goes east to Salzburg and through Austria to the east ending in Venice--but it's 9 hours 15 minutes.
It's over 300 miles by driving. If I was driving, it'd be for the incredible mountain scenery along the way--2 days drive.

According to ViaMichelin, if you take a route to minimize tolls, it's over a 10 hour drive, but for only about 25€ in tolls you can cut it to 7½ hrs. But the train does it in 6½ hours and you can look at the scenery.

Speaking of scenery, for someone from Colorado, Brenner Pass is pretty unspectacular. At about 4500 ft, it's wimpy. But it is pretty, and there are small towns all up the valley. BTW, if you take the train and look up at the underside of the high, high, high bridges, I don't think you'd ever drive it.

Of the 5 ECs, every two hours, only one at 11:34 goes directly to Venice via Verona. Two others, I think, end in Verona and two more go farther south, maybe to Bologna. There used to be one to Milan.

If you want it fast and unspectacular just follow the motorway Munich - Innsbruck - Brenner - Verona. Buy an Austrain toll sticker, pay the fee for the Brenner motorway and the Italian motorway - all in all €32.20 (in addition to gas and the hefty drop off fee if you rent a car in Germany and turn it in in Italy). But as Lee has sayed, that route is cheaper and less stressful done by train.

However, if you want it more scenic you could take a route across the mountains (four passes): Munich - Kufstein - Kitzbühel - Pass Thurn - Zell am See - Großglockner pass - Gailberg Sattel - Plöckenpass - Tolmezzo (where you reach the motorway Villach - Venice). It's conveniently done in two days and offers many spectacular views. On the south ramp of the Großglockner, south of Heiligenblut, there are a number of villages with many pensions that are ideal for an overnight stay.