So a little update on our situation. I contacted the Mieterverband, who were helpful in that they sent me a lengthy response. They found a lot of things that didn't make sense in the bill and said it was way too high as were my other bills that I'd paid over the years (I only sent those for comparison/reference). The problem is that my German isn't really good enough to understand it perfectly and it's three pages of information.

Can I reach out to someone here who has really good German to perhaps summarize it for me in English and help me with the letter to the landlord? The Mieterverband has included a generic letter to send but there are some bullet points for me to fill in, and my German really isn't strong enough. If anyone could help, I can pm you scans of the response to me and the generic letter. Thanks to anyone who can assist!