Itinerary Advise: Bavaria

Without knowing what your interests are, it is difficult for us to make suggestions. Do you want to see museums, historical sights (which era?), castles, art, churches, scenery, etc. ?
Heidelberg is an easy day trip from Frankfurt, with multiple trains all day long.

Garmish is my favorite, consider staying and/or dining at the Fraundorfer.

I wanted to make sure that you know about the Deutschland Ticket, often called the “D ticket.”
https://www.mvv-muenchen.de/en/tickets-and-fares/frequent-travellers/deutschland-ticket-d-ticket/index.html

This should provide you significant savings with your itinerary. 49 euros covers all regional and local public transportation throughout the country for one calendar month. It won’t cover high speed ICE trains.
https://community.ricksteves.com/travel-forum/germany/d-ticket-easy-to-get-and-easy-to-use

I would not increase my time in Fussen. I completely understand the appeal of Neuschwanstein for those who have not visited it. I felt it was a “must see” on my first visit. I found that it was a lot of effort to get there by public transport and the inside of the castle was a complete disappointment. This sentiment has been echoed by many others on the forum.

I'm not a train person, but it looks like you can get to Innsbruck in 1.5 hours from Garmisch-Partenkirchen. Would be a long day. Austrian Alps and a historic city.

Your interests need to be known. If it were me, I'd stay in Salzburg, and visit Berchtesgaden as a day trip. Or skip both and stay in Garmisch, from which you can do the castles, as a day trip. I'd skip Heidelberg, or make it a stop on your way to Frankfurt. And make the Middle Rhine Valley (maybe a cruise from Rudesheim) a day trip from Frankfurt. But thats just me.

Based on sites selected, I'm guessing you like doing outdoorsy things. If that's true, Berchtesgaden with a visit to Salzburg is better than the opposite (in my opinion). The Zauberwald offers a nice easy hike along a rushing river -- it goes from Ramsau bei Berchtesgaden to Hintersee.

At G-P, you can visit Partnachklamm, an impressive gorge.

I think if the castles are your reason for going to Fussen, 1 night is adequate, though there is good hiking in that area, too.

If you are interested in hiking, you could stay in Ehrwald Austria (just across the border from Garmisch). From Ehrwald there are many hiking options including some that will take you to a mountain hut where you can relax and enjoy a great meal. For instance, the Coburger Hut is just two miles from the top of the Ehrwalder Almbahn (ski lift) and is set between two alpine lakes with a view towards the Zugspitze.