Thanks for the post, MarkK!
The 81% performance record for Stuttgart > Singen > Zurich is impressive in light of the current average performance across Germany.
I used this same routing from/to ZRH Airport via Zurich HB for my recent visit to southern Germany with good results. Most of us with our eyes on places like Tübingen, Stuttgart, Esslingen, and the Black Forest probably think first of flying into FRA, but IME fares into ZRH are very competitive with those fares, and ZRH is, generally speaking, a nicer airport. And with the current consruction woes between Frankfurt and Stuttgart, ZRH seems a good go-to alternative for easy access to this part of Germany.
There are DIRECT trains from Zurich HB (1 hour to Singen, 3 hours to Stuttgart.) And from ZRH airport, one can use those same trains by tacking on a 10-minute local train ride up front to Zurich HB.
Singen, btw, while its touristic value is minimal, does have reasonably good infrastructure for travelers... affordable hotels (far more affordable than Zurich!!) as well as some ethnic restaurants, and at the new 3-story mall across the from the Singen train station, dozens of shops, eateries, etc.