Could you possibly try to ferment at a higher temperature? I only do
this at home, but I can get extremely sour starters but leaving the
sourdough at around 85-90 F. for an extended period, like 12-14 hours.

Can you get whole rye grain from organic food coops and grind it
yourself? This might at least give you a starter that you can use to
inoculate your German flour.

Ron