I rewatched the video at least 10 times now, trying to find my misake and I noticed that the dog bones also don’t look as pronounced as they should be, they should look like this: Measurements are off, but consistently - #17 by Tokoloshe
Or am I just imagining things now? At least your measurements are precise. 
I am milling it over and over in my head, the only thing that comes to mind is the YZ plate still moving somehow.
/edit: I figured out that it was the Diamantverzahnt endmill, a 1-flute cut it without problems, which is weird since I’ve always used those with the Primo. I am going to run more tests, but I think I had the RPM turned down too much. The picture I linked above was cut with the same settings that I used above, except the RPM! I will start at max RPM now again and work my way down, because the diamond bits are a) pretty cheap and b) leave a nice cut if you do it right. If nothing is working then they will be replaced by 1-flutes… 