I also like Garmisch Partenkirchen as a base and would prefer it as a base rather than Mittenwald. In fact, I am returning to G-P for a week in April and will do daytrips from there. For a wealth of information about the area, accomodations, events, restaurants, etc, check out their official website at gapa.de. It has tons of accomodations of all types listed: hotels, guesthouses, apartments, rooms, bnbs, etc. I combed through all of the accomodations on that website and found a good, two bedroom apartment at a gastehaus for less than 100 euros a day. If you are thinking of renting a car and returning it to G-P, then why don't you rent it there too, rather than rent from Munich and return to G-P and probably incur an additional fee? You can take the train or bus from Munich to Garmisch Partenkirchen. There is a FLIX bus direct from the international airport to G-P for 15 euros. If you go by train, buy a Bayern ticket if you are two or more. Once in Garmisch if you stay at an accomodation you are provided free local bus passes. We used the public busses in town everyday because of this benefit. From Garmisch there are lots of places to visit within an easy daytrip range. The Partnach Gorge is right there in G-P. Oberammergau and nearby Linderhof Palace, MIttenwald, Zugspitz, Eibsee, Neuschwanstein Castle, even a daytrip by train to Innsbruck are all easy doable places to visit. Lots of other choices too. Get yourself a good guidebook that focuses on Bavaria.
Salzburg is beautiful. We used public transportation and walked a lot while there. Since we didn't have a car we did a daytrip private tour up into the mountains and then on to Berchtesgaden. Since it was late November and snowing Eagle's Nest was closed but we visited some WWII sites in the area. I have not been to Hallstatt but it looks so pretty and my neice told me she liked St. Wolfgang even better than Hallstatt and it is closer to Salzburg.