In some countries, it's common to register a sole proprietorship in addition to a limited liability company and bill the company from the sole proprietorship to avoid double taxation. However, I suppose this would not be allowed in Germany.

I don't see how the German tax authorities would allow this since it would completely circumvent their rules about "disguised profit payments"?

https://de.wikipedia.org/wiki/Verdeckte_Gewinnaussch%C3%BCtt...