Itinerary help needed- Frankfurt- Nuremberg-Munich ( add Rothenburg)

It would be helpful to understand how you will spend three days in Frankfurt. In and of itself, Frankfurt is the least interesting of the places that you have mentioned.
For my own preference, without knowing your plans, I would head straight to Rothenburg after one night in Frankfurt.
The rest of the plan is fine depending upon how you plan to use these places as bases for other travel.

Logistically, Rothenberg fits best between Frankfurt and Nuremberg. Stay there overnight, not just a day trip if at all possible. Which city to steal a night from depends on how many sights you want to see in each place and how important they are to you.

how I could either reduce 1 night in Frankfurt or Nuremberg

Imo this depends on your interests and what you plan to do in / around destinations.

I can’t figure out the logistics where to fit it in

Map-based more Nuremberg - Frankfurt.

Journey planner of Deutsche Bahn for train and bus connections: https://int.bahn.de/en/

Flixbus is sometimes a bus option: https://www.flixbus.de/

Thank you for the advice, I think it makes the most sense to stay 2 nights in Frankfurt and 1 in Rothenburg before heading to Nuremberg. I might look at renting a car for a day or two as the train travel seems complicated.

Rent a car in Nuremberg and drive to RodT. Stay in Nuremberg.

A day trip from Nuremberg to Rothenburg is neither expensive nor complicated, especially since you would be leaving your bags in your Nuremberg hotel room and will not be toting them between platform changes. The one-way trip takes only 1 hr. 15 minutes. Two adults for Nuremberg > Rothenburg > Nuremberg: €23.90 on a VGN Tagesticket Plus, a day pass that is valid at any hour of the day:

https://www.vgn.de/en/tickets/all-day-ticket-plus/

Your planned day trip to Neuschwanstein is actually more complicated. It's good that you will be leaving your bags in Munich on this outing. But... Do you realize that morning departures will mean spending 2 hrs. 40 minutes one way between Munich and the final Neuschwanstein bus stop? (Round trip, that's 5 hours, 20 minutes of raw transport time.) THEN you get to begin your uphill walk to the ticket center and to the entrance (and of course the downhill walk on your way back to Munich.) This is a lot of logistics for a tour that takes less than 30 minutes!

A trip to Rothenburg is child's play by comparison. It's hard to imagine why one would go to the trouble and expense of renting a car just for Rothenburg. It's a small town that requires only a few hours' time. A half-day outing would be adequate... sample schedule leaving Nuremberg in the early afternoon...

Nuremberg > Rothenburg 12:34 - 13:51

See the town, have dinner, etc... 6 hours???

Leave Rothenburg as late as 20:05 or 22:05 for Nuremberg.

Everyone has their own tastes. We have spent 2 nights in Rothenburg several times. I prefer it to each of the cities that you have listed.

There are several places to visit away from the more touristy sites during the day (Detwang, the Vineyards, the Stadt Garten, the Rothenburg City Museum). Rothenburg quiets down at night and is at its most beautiful.

All the places that you have listed are big towns. Rothenburg is to my mind an interesting exception, especially if you do not just follow the crowds around the city square.

When I went to Frankfurt, I too thought it wasn't going to be as interesting as the other places.

Then I took a walking tour with Frankfurt on Foot and it changed my persepctive on the city.

It turned out that I enjoyed my time in Frankfurt more than my time in Munich.

a couple traveling in September for Octoberfest

Is this a beer trip? Are you doing both Oktoberfest in Munich and the Volksfest in Stuttgart? You may want to consider a day trip to Bamberg for their unique perspective on beer while you are in Nuremberg.

I will look into the walking tour in Frankfurt, thank you.

Beer is the theme, thanks I will definitely look into Bamberg!

in Bamberg it's Rauschbier.

Rauschbier.

Nice misspelling. In fact, it is made with the help of Rauch, but the experience can end up in a Rausch.