The first boards have arrived and they work! I also assembled a test board that contacts the pins of the tracker board and connects them to an Arduino Uno.
Uploading the bootloader worked well and the tracker program is also working. I need to swap some axises to make it track. Those are minor adjustments.
Furthermore I need to make a test program that tests all devices and creates a test report. This will be the last step to mass production capability.
Or to make it clear
****** S U C C E S S - We did it! ******