3 weeks in Germany, Austria, Italy, France - critique please

Why would you fly into Paris if you want to go to Salzburg 1st? Fly into Munich and out of Paris.

Berchtesgarten is fascinating, but maybe not ideal for 9 and 12 y/os.
The Red Bull museum hangar at Salzburg airport is great for F1 and bike fans.

...we changed a few things around.

Yes, you say you might visit Berchtesgaden. But otherwise, Germany serves as...

1) ...a place to zoom through at night on Day 1 and...

2) a place (Munich) for a one-day of sightseeing on the long journey from Ortisei to Strasbourg, France.

I'd consider replacing some of the time you had allocated for Germany in your previous itinerary. There are many nice options between Innsbruck and Strasbourg which migh offer up the "perfect mix of history, beautiful towns, nature, touristy and less touristy activities" that you seem to have in mind.

Mittenwald seems a good candidate and might be a good place to add - or to use as an alternative to big-city Munich. After dropping the car, the train ride from Innsbruck to Mittenwald takes only 57 minutes. And there's much more nearby. Linderhof Palace. Garmisch-Partenkirchen. The Fraundorfer Inn in G-P. The Zugspitze. Oberammergau. Highline 179.

That sounds like a well-planned, really fun trip! Your children might enjoy a salt mine tour in the Salzburg/Berchtesgaden area. Time in the mountains hiking or other activities might be fun for them as well.

bondi_zen,
Just one caveat. If you rent a car in Italy (Ortesei) and return it in Austria, you will incur a very hefty drop-off fee (500-1000 euros), Perhaps it would be best to rent the car in Austria, drive to the Dolomites, and return it in Austria, making sure the rental company will allow you to drive into and out of another country.
As to your itinerary, it has been quite a while since I have been in that area, so I can't comment, except to say I loved Munich and feel it should have more time. There are several things that would interest your kid: Deutsches Museum (science and technology), Marienplatz (glockenspiel), viktualen market, dinner at a beer hall (table-sharing, music, good sausages), the Residenz (art, jewels, very posh), Dachau (no, they are not too young...ours were 9 and 12 when they saw it...impressed and thoughtful, not traumatized).
I also love Vienna, and hope you can find time to visit. Glad you are seeing Salzburg.
I am guessing your flights are to and from Paris because of the distance of your journey and the options you have for the long-haul flights. It would be nice if you could spend a couple more days there than you have planned. But, c'est la vie! Bonne chance et bon voyage!

Sounds to me like the OP will rent the car in Salzburg and drop it in Innsbruck, both of which are in Austria, so no problem there.

Are 2 days in two Dolomites locations enough time? Or would 3 days in one area makes more sense?

Skip Cortina as it will have construction for the upcoming Winter Olympics. Ortisei would be great for 3 days.

Re: days 4,5, 6....If you are interested in Austrian history prior to WW1 , I suggest going to Bad Ischl where the Kaiservilla is located. This was the summer residence for the Emperor Franz Joseph. You can only see the interior via a guided tour. Pre-pandemic I did this twice, it was given only in German. It may have changed but don't count on it.

I'm curious if you have thought about flying into Munich and flying out of Paris. That would certainly give you more time and save you the expense of the night train to Salzburg, which is a short and easy commute from the Munich airport.

If you've already booked the tickets, then never mind, but if not, I would certainly look into it. Multi-city tickets are usually no more costly than a round-trip ticket

Bad plan imo:

• Too much backtracking in this plan. Waste of time and money. --> Resort order or places and change fly in and out airports.
• Two times Brenner Autobahn (A 13) in Austria to Italy is a real bad idea due to construction on the new Lueg Bridge. --> Replace Dolomites by another destination.
• Huge distances with 2 kids in tow. Less region hopping is likely more fun and experience time instead of driving around.

Good luck!

Thank you everyone for your responses, especially the points about the Winter Olympics and possible construction impacting our journey are good to consider.

To clarify:

-our flights are set - it was cheapest to fly in and out of Paris so that’s where we have to start and end our journey.

-we are thinking of picking the car up in Salzburg and dropping it off in Innsbruck after the Dolomites trip; and then will get another rental for when we’re in Strasbourg.

-looking at the maps, our drives shouldn’t be longer than 2-3 hours at a time. While we would like to minimise driving, our kids are used to these and longer distances, us being from Australia.

-we took out our earlier Germany stops (e.g. Lake Constanz) as that felt like too much driving.

-we chose Bad Ischl as our Salzkammergut base, looking forward to that.

Thank you once again everyone.

Two times Brenner Autobahn (A 13) in Austria to Italy is a real bad
idea due to construction on the new Lueg Bridge. --> Replace Dolomites
by another destination.

I have the same opinion but a different conclusion:
First go to the Dolomites and then to Innsbruck. You can drive from Bad Ischel via the Grossglockner High Alpine Road to Heiligenblut. Stay for one night in Heiligenblut or Lienz and then drive on to Ortisei for 3 nights.
But only do this if your children can cope with long journeys on winding mountain roads.

I was going to suggest what Paul636 said, but he beat me to it.