The 3/5 rule is easy to satisfy, as if you have a marketing presence you basically satisfy it. The 2/4 rule, which has been reintroduced, is if you have no employees or one client yu are liable for Sozialversicherung. In the past operating a Ltd gave you some protection, but not anymore. Employing your wife, or family is not acceptable. If you have two clients then it can't be the same owner. This is much more difficult to satisfy. So you really need to make sure you satisfy one of these criteria, to be sure. It is really nasty.
What is bad is that this development is very new so the consequences aren't clear. Up until last year it was "don't worry".