Germany and Italy

Twelve days in Italy? i would drop the Amalfi Coast (sadly); fly into Roma, visit Tuscany, Venezia, Milano, then finish in Germany.

We lived in Bavaria some years ago and loved going south to Italy.

I have found that renting in one country and dropping in another usually does incur a huge charge. Also, If you drive on the Italian Autostratas (equivalent of our interstate), the tolls are quite high.

I recommend taking the train down to Italy.

If you only have 12 days, I would pare down your Italian cities to Rome, Florence and Venice. Milan, in my opinion is not in the same class as those cities. Also, trying to take in the Naples area would take up more of your precious time.

Plan on minimum of 2 nights in Venice, 3 if possible, 4 nights in Florence and the rest in Rome. You could spend a week in Rome and still not do it all.

To do just justice to Naples, Pompeii, Sorrento, Capri and the Amalfi Coast, you would need four nights in that area. If you do go to the Naples area, you can do Pompeii on your own taking the train, but the other places are harder to do, especially the Amalfi Coast, without taking a tour.

If you insist on doing Milan, I suggest only one night there.

For an relaxed start take the train from Innsbruck (close to Garmisch) to Venice. Crossing Austria it goes right through the Dolomites mountain range, which makes this one of the best train trips in Europe.
And to make the rest of the trip relaxed as well, limit yourself to a few personal priorities. You could do worse than move yourself from Venice to Bologna. A very nice city and a good base for wonderful places around, which can all easily be reached by train. End this trip in Florence. Or in Lucca, from where you can do Florence as a day trip. Pisa with its international airport is only half an hour from Lucca.
Have Rome, Napels and the Amalfi coast for a next trip.

Two options

Day
01. Arrive Venice (3N)
04. Early train to Florence (2N)
06. Late train to Rome (3N)
09. Train to Sorrento/Amalfi Coast (4N)
- with days to Pompeii, Capri & Positano
13. Depart Naples

Day
01. Arrive Rome (3N)
04. Train to Florence (4N)
- with days to Lucca & Siena
08. Train to Venice (3N)
11. Train to Milan (2N)
13. Depart Milan

Yes, enjoy the relaxing TRAIN to Italy, where everyone can look out the window at the vineyards and mountains - including the one who'd be driving that rental car on the autostrada!

From Germany:
Garmisch - Milano - Venice - Florence - Rome - fly home from Rome
or do it in reverse...

Thank you for all your suggestions. Trying to figure out what's important for him to see on this bucket list trip. Mulling prices for open jaw tickets and insurance. Any suggestions for places to stay in these cities and Tuscany would also be appreciated. One thing I don't want to do is drag heavy suitcases up flights of stairs, we aren't getting any younger, (sigh), and I'm kind of small. Was looking at some Amex suggested hotels, but not sure if that's the way to go. I must admit I like full service hotels, as well as small villas with ground floor accommodations. I like the idea of starting in Italy, and leaving from Munich.

If not wanting to schlep around heavy bags, then read: https://www.ricksteves.com/travel-tips/packing-light/packing-smart

We are in favor of full-service hotels whenever staying for only 2-4 nights. If staying 5 days or more we sometimes consider an AirBnB, but the amenities of a hotel are very useful on a short stay!

For your bags, the answer is simple... Pack less. Read Rick's guide (posted above). You don't need half of what you're bringing. My wife and I have traveled for a whole month every year, three years in a row, with only one of these each:

http://road.cc/sites/default/files/cropped/preview_500/images/Timbuk2%20Q%20Backpack/Timbuk2%20Q%20Backpack%20-%20worn.jpg