Switzerland to Salzburg to Munich anniversary trip

In my opinion you have too many one night stops. My general rule of thumb is to never stay anywhere less than two nights. It’s exhausting to pick up and move every day..

The best place to visit in CH is Gimmelwald and the Berner Oberland. You should rent a car in Geneva and drive to Lauterbrunnen and park it. You would then take a cable car to Grutschalp. From there you'll take a scenic train to Murren and either walk to Gimmelwald (it's steep) or take a cable car. I would spend a minimum of three nights in Gimmelwald. RSs has a new CH guidebook coming out in May. You can download an electronic version from his app.
You can then drive to Zurich and drop off your car and catch a direct train to Munich (4h).
In Munich, take a 20m train to Dachau for a half day and take RSEs self-guided walking tour when you get back. You'll need to purchase his Munich Pocket guide that's available now.
If you have time, rent a car and explore the Bavarian Alps while staying in Munich. Every ten years there's a passion play in Oberammergau that is also an adorable town. This is the year it will be performed and has been in production since 1634 according to Google.

It is expensive to rent a car in Switzerland and drop it in Germany, especially if that German location is far from Switzerland. Your Swiss rental will have a Swiss vignette already attached, but you will need to purchase an Austrian at the border and attach that one as well.

For the most part, you don't seem to be going anywhere that requires a car, they are all well served by trains. Interlaken is not on the way to Bern, Bern is on the way the way to Interlaken. Also you seem to be going to a lot of cities in Switzerland, where the mountains are the real attraction, not cities. What are you planning on seeing in Geneva, Bern, and Zurich? With 6 nights in Switzerland, makes more sense for 2 nights Vevey/Montreux, 2 nights Lauterbrunnen/Muerren/Wengen, 2 nights Luzern.

I basically concur with Sam.

"1. Fly into Geneva, stay 2 nights" - Most people go to Switzerland to see mountains, not cities. And Geneva is not the best city in Switzerland, skip it.

"2. travel to Bern, go to Interlaken on the way?, stay 1 night" - You go through Bern to get to Interlaken. Bern (unlike Geneva or Zürich) is worth a day trip.
Do not stay in Interlaken, stay further into the Jungfrau region: Mürren, Lauterbrunnen, Wengen or Grindelwald.
Info about these places: https://jungfrauregion.swiss/en/summer/
And one night stops are a bad idea. You have travelling on two consecutive days and minimal time in the place. The Jungfrau region is the best part of Switzerland, and worth 3-4 nights at least.

"4. travel to Zurich, stay 1 night" - No, skip Zürich.

"At this point, we are leaning towards a car rental," - as above, renting a car in one country and returning it in another is very costly. And no cars in Mürren or Wengen. And cars are an impediment in cities like Bern, Salzburg, Munich.
Most of your route is train friendly. Except Zürich/Luzern - Neuschwanstein - Salzburg is not easy by train. There are some big mountains in the way that the trains have to go round.

Plan B: Mostly train. Do Luzern - Salzburg by train. this is a scenic run, see here (scroll down for video): https://www.seat61.com/international-trains/trains-from-Switzerland.htm#Switzerland-Vienna
Do Neuschwanstein as a day trip from Munich, either by train or rent a car for 1-2 days just for this bit.

Sorry, but I totally disagree with Gimmelwald being the best place to visit in the Berner Oberland. It's ok for an hour or maybe 2, but otherwise totally uninteresting and lacking in infrastructure. And you don't need to rent a car (definitely not) to get to Lauterbrunnen. Heavens no. Take the train all the way from Geneva to Lauterbrunnen. Change to gondola and then to train on into Murren.

And unless there's something you're just dying to see in Geneva then give it a miss and climb on the train and go to the Berner Oberland.

Thank you so much to everyone for their replies. You have caught quite a few items that I need to change on our itinerary and have given us quite a bit of ideas to reconsider. Thanks again - you have helped tremendously!

I hope you follow the seat 61 link Chris F. provided. Read about the Arlberg Pass. This is definitely on my list, You can go to Innsbruck and then decide north to Munich or farther east to Salzburg. It looks beautiful.