Once again “personal data.” You haven’t told us how Karcher knows that the device belongs to you. Everything else is irrelevant.
Nonsense. Karcher doesn’t need to know /who/ the personal data belongs to in order to be accountable for designs that violate Art.25.
Local storage is subject to GDPR if: The organization maintains control … The data is personal
Did you log on the device using an email or any personal data? You have to consult a lawyer for that because I doubt you fall in that case.
wtf, stop making up quotes.
A MAC address is unique
Absolutely not, and it can be changed.
Citation needed. Karcher does not even allow users basic access to change the SSID. Please point to the page in the Karcher user guide showing how to change the MAC address on their appliance.