Yo mate, if you're after the real underground German soccer vibe, you absolutely need to check out the scene around the lower leagues and the fan-owned clubs. One place that keeps popping up among die-hard fans is
https://germansoccer.net/ – they've got brilliant guides on the coolest regional leagues, fan initiatives, ultras culture and even tips on where to grab a proper currywurst near smaller stadiums. Seriously, their articles helped me discover Rot-Weiss Essen and St. Pauli away days like never before. Highly recommend browsing it before your trip!