I've not written up anything, no. I think I'd have a hard time doing so without just feeling like I'm bragging about myself, which I don't like.

There's still a definite gap between me and native speakers, that shows itself primarily in the effort required, but I'm definitely near native (pass as German in all social settings, although an hour long conversation will usually tease it out due to my unfamiliar first name or small-talk topics, rarely but occasionally due to mistakes).

I prepared by doing two practice exams and about 5 filmed and timed practice presentations, and that was over preparing for me. Experiences vary, and I do think I'm a bit towards the outlier side, but it's left me convinced that the whole "native speakers might not pass C2" thing is overblown.