Why do I always fall the supposedly easiest solution?
I checked the steering rack, tie rods, tie rod ends but could not find a reasonable explanation for the play (approx. 6mm on steering wheel).
Then I started with what I should have been doing right from the beginning - searching from where the problem starts all the way to the end.
Look what I found:
Short video as ZIP-file - I was not able to upload a video (The uploaded file does not have an allowed extension - I tried the usual?).
The u-joints along the steering column have play - does it make sense to go for the needle bearings only (if available) or is better to exchange the entire u-joint right away?
they are not that expensive, but you should also use UNF thread nuts and bolts as the clamping force they exert is more than standard metric thread bolts and nuts