Driving

All the other German skiers do it, so why not you? I would be more concerned with clear roads than scenery, which will come by and by. I'd stick to the Autobahn/Autostrada until you get to Fortezza, then take Rt 49 to San Lorenzo, then south on Rt 244 to La Villa, then Rt 32 to San Cassiano. Don't worry about scenery, it'll be there. Make sure you have snow tires on your car.

One-way cross-norder rentals are expensive. Trains (close) to destination are alternative and rent a car there.
Ensure rental car contract allows all countries.
Snow tyres are mandatory by law at expected winter conditions.
Buy vignette for Austria.

Most important: Check that all drivers' driving permissions (not only licenses) are valid in all countries. Violation costs insurance coverage in case of accident.

If you want to drive a car in Germany ensure the validity of your driving license in Germany and possibly also an IDP is needed to have valid driving permission in Germany.

Make sure you have snow tires on your car.

Snow tires are mandatory in Germany (or at least in Bavaria) in winter so there is no need to worry about that
I would probably just take the most direct route via Garmisch, Mittenwald and the Brenner Autobahn. If the weather is looking really bad, I would go fully on the Autobahn via Rosenheim. Conversely, if the weather is nice, you could go via Walchensee instead of Garmisch.

Snow tires are mandatory in Germany (or at least in Bavaria) in winter so there is no need to worry about that

This is not fully correct.
There is no general winter tire requirement in Germany, but instead a situational one (§ 2 (3a) StVO). This means that in winter road conditions, i.e. black ice, slippery snow, slush, slippery ice or frost, you are only allowed to drive on winter tires. The situational winter tire requirement is deemed to be fulfilled if winter tires are fitted on all wheel positions, i.e. on all four wheels. (Source: German ADAC)

Therefore not all rental cars have some, especially when the car is planned to leave the rental car stock soon or when they came in very new. This is also waht I know from my personal experience.

Thank you for the correction! It is indeed important to make sure that the car has them, because snow chains or winter tires are mandatory for sure in Südtirol, and snow chains are a real pain....
If you must resort to chains, carry cheap gloves.

We lived in Augsburg, Germany from 1988-91 and went skiing every year between Christmas and New Year. We went to either Garmish, Germany or Oberdorf, Austria. These were places not deep into the Alps, but one year we had to buy chains just to get to Austria.

Personally, I would not make the assumption that you could do this. However, I am from Georgia and had little experience driving in winter conditions. I think you would likely need chains, but I have heard that driving long distances with chains can be a problem, since if parts of the roads have been cleared, then your chains hit pavement and wear out before you need them again on ice.