Bavaria/Austria/Italy Itinerary Help!

The thing is if you're going to try and see the area around Munich you're not going to have time to go to Venice. As noted, it's a 6-7 hour train ride. That's the better part of a day. And then you spend another going to Salzburg or Garmisch? Two whole days gone from your trip. And one day in Venice is pretty much useless unless you just want to say you've been there.

Slow down.

Munich and Garmisch in a week is doable. So is Munich and Salzburg. Or even Munich and Venice, but I wouldn't recommend it. Stay in Bavaria and/or Austria and enjoy the beauty as the season changes.

Last, if you don't know how to drive a stick shift (most rentals are) in snow (good chance you'll see some), than this is not the time to learn.

I did not count time we are already planning to spend in Munich, so that is not part of the equation. Also, I do know how to drive a stick in snow, grew up in Buffalo :)

"What are the best places to focus on in a couple days when based in G-P?

I assume you narrowed your choices to G-P and Salzburg because you have researched those areas and were attracted by certain sights and places in the immediate area. What's best depends on your interests, which are...? Which places are tugging at your sleeve? You could spend some time in both, or the whole week in one place if your interests match up better with just one.

Are there areas in this part of Austria (Salzburg) that are more worth spending time in vs those around G-P (eg, Hallstatt)?

Hallstatt is picture-perfect but won't demand much of your time. Much closer to Salzburg are Werfen and Hohenwerfen Castle. Into cars? What about the Porsche Museum? Then there's the Eisreisenwelt. Have you looked into those? These Austrian options are all quite different from what you'll find in G-P. Not sure how you would compare. Then of course there's Berchtesgaden and more just across the border.

In between Munich and Salzburg is the impressive Herrenchiemsee Palace (on an island in the lake of the same name) which might interest you.

https://www.breitbrunn.com/Fotos/Breitbrunn-Luftbild.jpg

https://www.schloesser.bayern.de/englisch/palace/objects/hch_ns.htm

And right near there in Aschau you can catch a lift up to the Kampenwand peaks for a view and some hiking.

https://kampenwand.de/index.php?rex_resize=835h__panoramakarte_sommer.jpg

http://www.chiemgau-wandern.de/images/pictures/w7de111120b2b000893989862a46d7f3/w7e081210161900025609dcff1eea1a3/Sonnenalm-%281-von-1%29.jpg

http://q-ec.bstatic.com/images/hotel/840x460/439/43953086.jpg

In between Munich and G-P you might enjoy a visit to the medieval town of Landsberg.

Landsberg photos (scroll down) - https://www.stadtbild-deutschland.org/forum/index.php?thread/1974-landsberg-am-lech-galerie/&s=d2aacc2f01314ce15f5fed3ae6cfec94df76ba83

https://www.tripadvisor.com/Attractions-g641965-Activities-Landsberg_am_Lech_Upper_Bavaria_Bavaria.html

Andechs Monastery (and brewery) is covered in Rick Steves and is just a ways outside Munich and also on the way to G-P.

I don't think most people with a week would find room for a visit to Venice.