There seems to be a ZVV-Flextax combo but I don't know if it is useful for me or where I can buy this.
Sorry, forgot to address this.
The Flextax / ZVV combos are only for monthly passes.
In any event, as Singen is not part of the Flextax coverage area (Schaffhausen and surroundings), Flextax tickets aren't of any use to you.