But i guess if we would implement this in the correct location the product would be auto done because the product is build from a generated repository right?
Right. Signing after the fact is source of errors as you mentioned (brining incorrect checksums and so on), artifacts need to be signed as soon as they're produced.
At the moment, I believe your approach with building your own Orbit-like to repackage and apply a signature directly when producing those artifacts to consume them later is the best possible one. I don't foresee an obvious possible improvement to implement in Tycho in short-term to improve this story.