Driving or taking the train (from Switzerland to Germany and Austria)

"One week" is not nearly enough for everything, no matter how you travel, and vague as well. Maybe you mean 7 days total, or 7 on the ground or 8 days including flights, or??

@Russ

Thanks for the reply, it is quite rushed, hence I was wondering if there's a better plan to these.....

We have 15 days total, landing in Zurich on Sat night - Day 0, so 14 days in actual. Our flight departs on the following Saturday evening (from Frankfurt).

The Switzerland leg of the trip has been somewhat finalised, so we leave Switzerland on Day 7 from Lucerne. Which leaves Day 7-13 for Germany and Austria (Day 14 would be the day we flight back)

I do know that it is a lot to visit in the 1 week we have left, to cover 2 countries...

With this plan and your short time there's no chance of "covering" Germany, much less Austria. The most sensible change you could make to enhance the efficiency and economy of your trip, given the airports you have set in concrete, is to reduce the amount of time you are planning to travel between destinations - and to keep your travel mostly between your airports. So... what is there in Germany between ZRH and FRA that might be similar to the destinations you had in mind?

Salzburg is a handsome historic city. But so are these places...

Tübingen
Esslingen
Freiburg
Metzingen is not far from Esslingen and Tübingen. Also nearby is Sigmaringen - a recent stop for me in this area.

Black forest towns like these aren't unlike Rothenburg:
Gengenbach
Schiltach

Stuttgart, like Munich, is popular for visits to its automobile museums.

And further north toward Frankfurt are places like...

Heidelberg
Guttenberg Castle
Michelstadt

There are many other good options for seeing Germany in this area as well. But you have only a week.

Reducing your ground travel will also permit you to use fewer base towns. With 9 people to house and the difficulties involved with that selection, I would attempt to find two locations at most. Then do day trips from these places.

While I'm fully aware that Rick Steves' advice for seeing Germany is focused on Bavaria to the near exclusion of the rest of the country, and that this probably was NOT the advice you had hoped to receive, I am certain that a very nice visit to this part of Germany can be quite rewarding. AND... if your in-laws or others sense that you are swindling them out of seeing two countries instead of just one, it is very easy to include a short day trip from Freiburg or Gengenbach into the French Alsace, where places like Colmar, Eguisheim, and Strasbourg have much to offer.

Hallstatt, Metzingen, Munich, and Rothenburg would be on the list of places my in-laws would want to go to, hence it would be best to keep them

Since it sounds like the in-laws might be a major force here, I took a quick look at your options, leaving out Metzingen, the outlier, and including some of your stated goals as well, to see more precisely what sort of ground-time you would be investing to go that far afield. I've also omitted Mittenwald and Nuremberg. You'd be starting from Luzern, not Zurich... Estimates per schedules at DB...

Day 7 Luzern > Salzburg 6.5 hours (scenic train ride, see the Seat 61 site for description)
Day 8 in Salzburg
Day 9 Day trip to Hallstatt by rental car, 2.5 hours rt + car pick up and drop off... or 4 hrs. rt by train
Day 10... to Germany, finally! Salzburg > Neuschwanstein 5 hours
Day 11 Neuschwanstein > Munich 2.5 hours
Day 12 in Munich
Day 13: Munich > Rothenburg > FRA airport 6-6.5 hours.

Total ground travel time = 23-25 hours

Seems to me that Munich gets somewhat shortchanged with this plan - and Germany more generally does too - but hopefully you have modest interests in Munich. Most of the Rothenburg fans here would also decry the daytime stopover I suggested, saying you must sleep there and must take the "Nightwatchman" tour (which actually might be your only personal contact with a real German person on this whole trip, actually, since this plan would leave you with so very little time in Germany.) So I guess if you agree with those people, you'd need to steal a night from somewhere else. I would not know where. The best move might be to cancel Neuschwanstein and Hallstatt on the basis that the Swiss Alps that you've already seen are so much more impressive than the German ones (and that N'stein is a waste of time, IMO) - and to instead visit some nice German destinations. Nuremberg and Bamberg, near Rothenburg, spring immediately to mind.

Anyway, I still prefer my previous suggestion, which shouldn't require more than around 12 hours of ground travel... Here are the basic legs with approximate train travel times for the Luzern > FRA airport itinerary:

Luzern > Freiburg 2 hours
Freiburg > Stuttgart 2 hours
Stuttgart > Heidelberg 1 hour
Heidelberg > Frankfurt Airport 1.5 hours
(Other misc. hours for side trips to Esslingen, etc: ?? 5 hours?? just depends...)

welcome to the Forums

A couple of practical considerations...

Are you travelling long haul to the holiday, in which case you might consider the impact that jetlag might have? Or only a short hop from the UK?

As you're flying, what provision have you made for the carseat issue with the kiddoes? Are you aware of the International Driving Permit for each driver, or do you all have UK driving licence?

With two vehicles, how will you stay in touch, especially if you get separated? Do you know about the no touching phones while driving laws where you will be?

I hope that all in the group will be happy with the itinerary - I think you are courageous tackling such a heavy driving trip.

Have you considered trains? Or a driver and a minibus?

OP, I think you have received some great advice here.

I love Salzburg and am no stranger to aggressive itineraries, but I would not recommend this route even to two experienced adult travelers with similar interests and energy levels. With the complexities that come with a multi-generational group of 9 I fear it is a recipe for a tough week.

As Russ thoughtfully points out, this is 24 hours of travel time not to mention reservation counter time, bathroom/rest stops, hotel check ins, etc. A lot of your precious vacation will be consumed by logistics even if everything goes flawlessly.

The region between your two airports has a lot to offer. Castles, cathedrals, quaint villages, vineyards, unique cuisine, nature. You can easily occupy 7 days here. I will add Speyer and possibly Mainz/a bit of the middle Rhine to the list.

Alternatively, you can aggressively prune the side trips. Eliminating Hallstatt and Neuschwanstein will save a lot of time. Salzburg, Munich, then work your way back to Frankfurt.