Thanks for your Answer.
I am using Tiki 21 LTS. Forgot to mention that. Sorry. When it's available, I'll upgrade to 24 LTS.
Nobody needs to open all the levels - but can, if wanted. For now, I just added a custom Module and put all the links to the pages I have by now (only 10 in 4 levels). Levels are just indented using html & n b s p ;, because + does not work - even with wiki syntax enabled. Now I'm only missing the little triangle to fold it all. Maybe that's possible using js.
This many levels may not be super user friendly, but it's a way to sort a knowledge base similar to wikipedia (just more managed and very specialized). The only other solution I can think of, ist just to scrap it altogether or use a special page for it. Maybe that is a solution for later times... for now, I want the navigation bar.