"Help a Family of 6 Heading to Munich & then Salzburg for 1st time!!"
OK, I do see you need some planning help. It might not be the help you expect, but having been there and done that many times, I'll give you my best advice.
"We land at 7 AM so have to get a car..."
A general rule of thumb is that getting behind the wheel in an unfamiliar driving environment, with signage in a different language, different rules of the road, and a different driving culture, especially after an overnight transatlantic overnight flight (possibly resulting in a lack of sleep, drowsiness, jet lag, etc.) and for a 2+-hr. drive, is an invitation to impaired driving. Also, there is no sound reason for having a car while in Munich or visiting Dachau. A more prudent planning strategy for Day 1 would put the 6 of you on a simple and direct 30-minute train ride into central Munich; after that, you check in for 3 nights (or so) and do your Munich/Dachau sightseeing over those first days, using public transportation.
If you want or feel you need a car for your tour of N'stein, H'schwangau, and Linderhof trip (all are palaces - not a castle in the bunch, btw!) you could pick up the car on Day 4 (or perhaps the evening prior.) Rather than travel BACK to Munich, you could just drive straight on to Salzburg for your first night there.
On your last full sightseeing day in the area, you'd leave Salzburg (or wherever you end up that day) in the pm and head straight to MUC airport for a final hotel night there.
"...anything else a must?"
I don't consider anything on your list a must for everyone. Go ahead and see what interests you most, but DO follow up with some more research. You know yourselves best. Right now, I don't see much "personalization" of your itinerary. It's basically just what Rick Steves thinks is cool. What interests do YOU have? Munich does have some good museums of all kinds, including the automobile kind. The Alpine destinations - Füssen, Oberammergau, Garmisch-Partenkirchen, and Mittenwald, for example - are much more than just afterthoughts near palaces built by an eccentric Bavarian Monarch. Munich itself has an excellent palace experience - Nymphenburg - with more to see than any one of the alpine palaces. With some more looking, you might find that some of your first choices are replaceable. Here are a few links I have handy to get you started.
https://hastingshouse.typepad.com/hastings_house_us/2007/04/mittenwald_germ.html
https://hastingshouse.typepad.com/hastings_house_us/2006/05/hallstatt.html
https://hastingshouse.typepad.com/hastings_house_us/2006/07/berchtesgaden.html