5 Day Southern Bavaria and Garmisch Parenkirchen Itinerary

Between Rothenburg and Frankfurt is the town of Miltenburg. In many ways it's better than Rothenburg as it isn't nearly as crowded. Do a googl search and see what you think of it.

Also taking advice on if these activities seem like the best
highlights.
Welcome to the forum.

Are you flying into FRA airport on Day 0 for this trip?

You would be logging a minimum of 1,000 km for this trip. The German Alps are very nice but they do not begin to compare to the Swiss alps. If I had not yet seen the Swiss alps (I don't know if you have) and were going to commit to driving that many miles, I would head to Lauterbrunnen, in the jaw-dropping Bernese Oberland. The mileage will be the same.

Personally, however, I would not commit to that many car miles. With your stated interests, I would instead choose to visit places on...

The German Fairytale Road
https://www.germany.travel/en/nature-outdoor-activities/german-fairytale-route.html

The German Castle Road (which includes Rothenburg)
https://www.germany.travel/en/nature-outdoor-activities/the-castle-road.html

The Romantic Rhine - as this website asks, "Why wander so far afield, when the good things are so close at hand?" Here, you can STAY in a castle if you wish.
https://www.romantischer-rhein.de/en/castles

Lots of romantic, fairytale/old-world towns on the Rhine:

Bacharach
Braubach
Rüdesheim

These places are largely within striking distance of Frankfurt.

You can visit Linderhof Palace on your way from Füssen to Garmisch on Day 2

Take scenic road Reutte(Austria)- Lake Plansee-Linderhof Palace -Ettal Monastery-Garmisch a 1,5 hr drive

ca. 1hr from Füssen to Linderhof and 30 minutes from Linderhof to Garmisch

A stop in Ettal recommended but you can do that also on your way from Garmisch to Oberammergau on Day 4

Day 1 Fussen, visit Neuschwanstein & Hohenschwangau, stay in Fussen
Day 2 Drive to Garmisch-Partenkirchen, visit Lake Ebisee and Zugspitz, stay in Garmisch area
Day 3 Garmisch Partenkirchen cable car to another mountain or hike, stay in Garmisch area.

Stay all nights in Fuessen. Very charming old town.
Day 1 Neuschwanstein and Hohenschwangau. Walk the old town.
Day 2 Drive 30 minutes to Ehrwald, Austria and go to the top of the Zugspitze. You’ll go to the exact same spot the Garmisch trip goes to, as the Zugspitze is on the border of Austria and Germany.
Visit the Ehrenberg castle complex at Reutte, by Fuessen.
If you have time do the Highline 179 in Reutte.

www.zugspitze.at

https://www.ehrenberg.at/de/ruine-ehrenberg.html

https://www.highline179.tirol/en/

Day 3 Another cable car mountain trip could be the Tegelberg Cable Car and there’s a Luge Ride also. Right by Neuschwanstein.

https://en.schwangau.de/sightseeing/the-tegelberg-cable-car-and-summer-luge

On the drive to Rothenberg ob der Tauber, stop and see the amazing Wieskirche.

https://www.wieskirche.de/en/index_en.html

We stayed twice at the Hotel zum Hechten in Fuessen. Loved the hotel, loved the old town.

https://www.hotel-hechten.com/en-index.html

https://en.fuessen.de/

Why not fly into Munich. Much closer to what you want to see

If you end up in Ehrwald, Austria it is a beautiful spot in the Zugspitze Arena. You are surrounded by Mountains with the Zugspitz and Sonnenspitze prominent. Not a whole lot going on in the town itself though. You can take the Ehrwalder Almbahn up to the Ehrwalder Alm. One of my favorite light walks is across the golf course between Ehrwald and Lemoos. We usually stay at Hotel Spielmann, but it can be pricey.
We like to stay at Staudacherhof in Garmish.
https://www.bergfex.at/ehrwald/webcams/c816/