I've been reading through your posts about your basement mixing room and I wondered if I could ask you some questions from time to time about it? I also Live in Berlin and am intending to make an acoustically treated and isolated room in a basement, similar to yours.
It also has 210cm high ceilings, but I think it's a bit smaller than yours, around 20 sq meters. The main use will be as a band practice room, but I'd also like to do some drum recordings in there, so it's really important it's isolated. Luckily, directly above is a room that I am renting, so it's only the neighbor above my room that I have to worry about. So i hope it will be a little easier than yours to isolate.
I don't get the keys to the place until the 1st October, but I thought I'd message you and say hello anyway.