One day Black Forest route assistance

Can't help with the Black Forest drive, but I highly recommend Mainau Island on the Lake Konstanz end of the trip. The gardens should be lovely in May.

Drive north first to Gengenbach for a short stopover.

http://www.orte-bw.de/grafik/uploads/10557_2010_1040.JPG

Then to Gutach (Schwarzwaldbahn) for a visit to the Vogtsbauernhof Freilichtmuseum for a glimpse of Black Forest life over recent centuries.

https://www.black-forest-travel.com/places-of-interest/vogtsbauernhof.html

Triberg is a popular stop (waterfall, Black Forest Museum, Hubert Herr cuckoo clock workshop.)

Further east you might enjoy Villingen.

http://www.tourismus-vs.de/en/views-of-the-city/sights/villingen.html

Lindau is an island-town in Lake Constance. Gorgeous place.

There's much more to see and do in the Black Forest than this, but your time for the above is too short as it is. IMO it's a shame you're rushing through so quickly just to get to Fuessen. Seems like every tourist within a day's drive of Fuessen wants to go there to see the castles which are not castles.

I would do this route by train myself. The Black Forest railway between Gengenbach and Lake Constance is an engineering feat and one of Germany's most scenic train rides.

https://www.black-forest-travel.com/places-of-interest/black-forest-railway.html

Thank you for your help. If I had more time to explore every inch of the planet I would, particularly the Black Forest, unfortunately I have 2.5 days to get from Lucerne to Frankfurt via as much of southern Germany as I can.

Our German hosts took us on a driving tour of the Black Forest this past June. They were disappointed that some of the routes they had chosen were closed due to road construction. You might want to check before you drive in May.

If I had more time to explore every inch of the planet I would ...
unfortunately I have 2.5 days to get from Lucerne to Frankfurt...drive
the entirety of the Romantic Road ...Any thoughts are welcome and
appreciated.

OK, having spent considerable time in the Black Forest and also Bavaria, where I toured the Romantic Road using public transportation (buses for a large part), I would recommend staying in the Black Forest. That is a more direct route to Frankfurt.

I'm not a big fan of the Romantic Road, particularly the Road itself. Using ViaMichelin and going from town to town, mostly on the Road itself, the trip by car would take almost 8 hours - a day's journey by itself - and it's just a winding, two-lane rural road filled with truck traffic, tour buses, and farm vehicles. It's really nothing special to drive. If anything is special, it's the towns along the road, particularly Landsberg, Donauwörth, Harburg, Nördlingen, Rothenburg, and a few others. But you probably don't have enough time to adequately see all the towns as well as get to the Road and from it.

Instead, I would go from Freiburg (and you can do this by train and have more time to watch the scenery) to Titisee to Donaueschingen, the head waters of the Danube. From Donaueschingen, go on the Schwarzwaldbahn to Triberg, then down the Gutach to Halsach and the open air museum. From there go up the Kinzig through Alpirsbach to Freudenstadt (a good place to spend the night). From Freudenstadt, go over to the Nagold river and down it to Pforzheim, stopping on the way in Calw, home of Hermann Hesse. From Pforzheim, go to Stuttgart and follow the Neckar down to Heidelberg, then to Frankfurt.

I think this route would be more direct and would actually have better scenery than the Romantic Road. By the way, at one time or another I have been on almost all of this route.

"I have 2.5 days to get from Lucerne to Frankfurt via as much of southern Germany as I can."

I agree with Lee's comments - especially his alternative area of travel and about the flawed scenery along Romantic Road... "it's just a winding, two-lane rural road filled with truck traffic, tour buses, and farm vehicles. It's really nothing special to drive..." That's how it was when I drove it as well. The places in/near the Black Forest and the Neckar River Valley are superior in terms of scenery. Some further suggestions in/near the areas Lee has suggested:

Glatt Castle in Sulz
University town of Tübingen
Tübingen's town hall and market square
Hohenzollern Castle, south of Tübingen
Ladenburg, near Heidelberg
Hirschhorn w/castle, near Heidelberg
Michelstadt and Miltenberg, near each other on a detour between Heidelberg and Frankfurt.
Map of the German Half-Timbered House route in and surrounding the Black Forest

IMO it's futile to attempt what you don't have time for and to drive right past all these nice places much closer to your route north.

For only having 2 1/2 days to get to Frankfurt, I would spend it all in the Black Forest ( and perhaps adding time at Colmar and Strasbourg in Alsace ), ending in Offenburg where I would get the ICE train to Frankfurt.