Den Hinterbau gabs gegen teueres Geld als Chrashreplacement ( blöden Zeitpunkt erwischt. Corona + kaum Alternativen)
Und auf meine Anfrage zum Hauptrahmen bekam ich diese Antwort:
... Those hairline cracks are in a place where the frame is not going to fail, so it's OK to continue riding.
If you don't use the internal routings and you have access to an Aluminum welder, the routing holes could be welded solid, or covered with a plate. If not, you can just continue riding and check them from time to time, if they have gone bigger. As I said, the frame is not going to fail there while riding.