Theres a nice LawTech company in Berlin operating currently where they will take the case for you, in trying to get your rent back to market level, and in Berlin they backpay the difference.
The company Conny.De/en upon a win, take 3months of the rent per month they save you going forward (and in retrospect ).
Plenty of people with wins in my friendship group
Announced a day after this years amount was due.