I am very happy today, because I finally managed to align my drive without the use of oscilloscope or any other equipment. After many attempts of trial and error I was lucky enough to find the "correct" position of the head. I tested the drive with some original games, cpm disks, and copy disks. All run without errors.
In order to do that I had to manually stop the motor around half way (red arrow) in order to fully access the stepper with the screw (blue circle). I also had as reference a working drive in order to see the correct position.
After that, CAT and a lot of (R)etry with different positions of the "ring" (blue circle). At some point the disc was read and everything worked.