I had a look at an April post of yours to put your current ideas in context... From a Baden-Baden base, you planned to day trip by rental car north to Worms and back...
"...to visit a small town that's not far from Worms to explore a few family roots."
(Worms is well to the north of Baden-Baden, roughly 2/3 of the way to FRA airport - a round-trip drive of 3 hours.)
In that thread, you shared plans to also visit the Technik museum and Speyer (Jewish family history) on that same day - correct? This will indeed be a long day.
On the NEXT day, your plan - even though you use the phrase "day trip" - is to take a one-way journey, leaving B-B for good and visiting Black Forest places that "...are south and east of Baden Baden" and continuing east via Lake Constance "(making it a longish day trip)" and finally landing in Füssen. This is the "scenic route" you speak of in the current thread, correct?
"...it looks like we'd be best off with a car on the way to Fussen if we want to go the scenic route."
So whether by car or by train, this is indeed another VERY long one-day journey, one that I would try to avoid myself, especially if I planned on some nice visits on the way (Gengenbach? The Black Forest Museum in Gutach, maybe? Triberg? Schiltach? Villingen? Donaueschingen?)
Have you considered doing a one-nighter on Lake Constance instead? Meersburg? Lindau?
The TRAIN route from Baden-Baden to Lake Constance is known as the BLACK FOREST RAILWAY and passes through many nice Black Forest destinations; it is one of Germany's most scenic rides. So I will assume you have good reasons - other than scenery - like the stops you plan to make? - for keeping the car on this day.
LINDAU isn't just a pretty face - it's where Europcar, Hertz and Sixt (All 3 work with Autoeurope) are located and could be just the spot to drop your car.
The next morning, you ride the train from Lindau to Füssen. Because this trip is entirely within BAVARIA, the BAVARIA ticket (€34/2 adults) will get you to Füssen.