From http://thesavvybackpacker.com/long-term-travel-europe/

The easiest ways to extend your trip past 90 days is to visit both Schengen and non-Schengen countries. Once your 90 days is up, travel to the UK, Ireland or many of the Eastern European countries that aren’t part of the agreement. Once you’ve spent those 90 days there you are free to return to to the Schengen area for another 90 days. You can keep repeating this process until you run out of money. - See more at: http://thesavvybackpacker.com/long-term-travel-europe/#sthas...

You can only be in Schengen zone for 90 days out of 180, so expect to spend some in eastern europe or UK/Ireland. You may also qualify for the german freelance visa:

https://medium.com/@imcatnoone/how-to-get-your-german-freela...