Thanks everyone, I got it up and running with the REW / UMIK measurements and the new MultEQ App measurement as basis. I also discovered at least two errors I made the first time, while making the filters: Used "A+B" instead of "A*B" and the filter txts contained headers/remarks although the box was checked.
The only thing that didn't work as intended (?) was getting the custom volume levels and distances in via CSV and browser script. I had to do that manually in the MultEQ app / AVR menu afterwards.
Sound stage and depth are amazing now, but I could use a little bit more bass and a tad less treble, so I am going to try again with another target curve... sometime.
@OCA if it's not too much of hassle, it would be great if your offer still stands regardless and you could take a brief look at my files to make sure everything is in order?
have a nice weekend,
Timo