Day trip from Frankfurt to Rothenburg

Part of your difficulty may be because Rothenburg ob der Tauber (RodT) is so far away from Frankfurt am Main. A good two hours there and another 2 hours back, or 3 each way on the trains.

Can you arrange to get there on your own and arrange to meet a guide there.

It is sort of like trying to find a guide in Boston to take you to and show you Burlington Vermont.

Rather than Rothenburg, a reconstructed town that is 3 hours away by bus, consider something closer and more original. Limburg, Marburg, Gelnhausen, Büdingen, Michelstadt, Eltville, Seligenstadt, or Idstein.

If your heart is set on riding that long distance to Rothenburg, just take the trains there and get a guide in the town. See what the TI has to offer. The Viator and Get Your Guide tours are probably with ETS and you will spend 6 hours on a bus going and returning.

I don't know if you are aware, but despite warm support from our host's books for Rothenburg, many posters here consider it overtouristed and not unique. (I haven't been there.) I can certainly vouch for Idstein. You might care to read the Fachwerkstraße section of this trip report:
https://community.ricksteves.com/travel-forum/trip-reports/baltic-germany-fachwerke-landesgartenschau

I try to keep daytrip trains to 1 hour or so. I'd make an exception for Cologne from Frankfurt, but that's not a substitute for Rothenburg. I mention Cologne only because it's only 1:30 from Frankfurt, and most of its attractions are walkable from the HBF, which is unusual for a big (postwar rebuilt) city. I trust you are already visiting the Middle Rhine for a boat ride.

I spent two nights in Rothenburg in September, 2019. I thought the town was adorable and it was a highlight of my trip. I am so glad that I spent at least one night there, because the town is magical once all of the day-trippers leave, plus I was able to go on the not-to-be-missed Night Watchman tour.