Thanks a lot for your very good explanation. Makes sense and I am afraid I have to go through these tedious steps...

One more question: Is it possible to extract the unsigned TX HEX string from my signed TX HEX string? (ideally by omitting some parts or be using a command line tool or so). If yes, I can save substantial effort by not needing to re-build the entire TX again (esp. because I have many of thoise TXs with 1 input and N (N=many) outputs).

Or is the non-standard thing that casues my problem already included in the unsigned TX and not in the signing part?