In the meantime I have added this function in my sketch (without the ....... dots) and also added the command
user_init();
as first line in the setup.
Sketch compiles and I flash it but sadly I still get all outputs from Serial.print functions to the normal Rx and Tx .
So there is something more going on which I do not understand
edit: also tried it the way exactly as you supposed but that gives me the same result.