Rental Cars and Government Holiday Issues

Your itinerary seems a litlle "wuschig" to me, sorry German slang for unsorted, double ways. Also: I would give yourself more time for experience than traveling only.

Starts with airports: Choose MUC or NUE (for arrival or departure, not both) instead of FRA. Less travel way for your itinerary (you will save up to 1 day).

In my head i would do it the other way around to have the "secure" train connection at the end.

FRA or NUE: If NUE, enjoy Nuremberg first evening and part of morning.
Take rental car.
Car to RodT. If NUE check factory outlet Adidas / Puma in Herzogenaurach.
Drive Rothenburg down to Füssen (Romantic Road)
Neuschwanstein area & Reutte
Zugspitze & Garmisch
:::
Alpine Road to Berchtesgaden
Freilassing to drop-car
Salzburg
Train to Munich
Munich
Train to MUC airport or to FRA if you have to

Or put even Munich in the middle after Zugspitze (avoid double way). Drop car there, go by train to Berchtesgaden and Salzburg, also fly-out from there.

Be aware that you will need motorway sticker if you want to drive on Austrian motorways (Autobahn).

Hope that helps. Sorry for being bit direct - no negative intention.

Thanks for the reply and suggestions. No offense taken for directness. Always much more appreciated actually. :) We are booked for the Frankfurt flights - both in and out, so no changing that.

As well, I know there is a lot of "travel" included, but we are fine with that. As Americans, we only get a limited number of days a year, so we can't always have the ideal vacation and see as much as we want. Trust me, if we had weeks to travel, we absolutely would do so. We're used to long days while traveling with a variety of activities. We've accepted we don't really rest while on vacation. I understand that is not ideal (or even acceptable haha) for everyone.

I've thought about reversing the itinerary, but we have an overnight flight from Frankfurt, so the first day in Munich is essentially a wash with an early bedtime. If we shift to putting that to Nuremberg/Rothenberg, I'm worried that would be a wasted night for the most part.

If we would reverse the itinerary, my last question still applies -- Is taking the train from Munich to Frankfurt on the day of a 5 p.m. flight something most Germans do without hesitation?

If we would reverse the itinerary, my last question still applies --
Is taking the train from Munich to Frankfurt on the day of a 5 p.m.
flight something most Germans do without hesitation?

Of course I do not have a representative survey but most people trust DB. In the last months long-distance trains had some severe "on-time" issues. The ratio of trains arrived as planned +5 minutes were down to 70-75%.

On-time ratios per month here - click on "DB Fernverkehr" above graph:
https://www.deutschebahn.com/de/konzern/konzernprofil/zahlen_fakten/puenktlichkeitswerte-1187696

Redline shows value mentioned, black show on-time + 15 minutes.

One minor comment w/o regard to other replies. Berchtesgaden is practically a suburb of Salzburg, so you could easily take a bus or short train ride to get there, and rent the car there (i.e., in Germany).

WHAT STAN JUST SAID.

Lots of buses every day direct to Berchtesgaden from Salzburg.

If B'gaden doesn't have the company you want, it's a 1-hour, 9-€ train ride from B'gaden to Freilassing on Tuesday.

When you are in B'gaden you get a free local transport pass.
"With the Berchtesgaden guest card, overnight guests in the Watzmann and Königssee holiday region have free and environmentally friendly transport on most bus routes."
https://www.berchtesgaden.de/en/general-information/maps-local-transportation

it's a 1-hour, 9-€ train ride from B'gaden to Freilassing on Tuesday.

To pick up a car I'd take bus #24 (from Südtiroler Platz, in front of Salzburg Hbf). It stops near the offices of the big rental companies: Europcar: stop "Salzburger Platz", Avis and Sixt: "Sonnenfeld" (terminus of bus #24),