update: So I managed to get it working again. HUGE thanks to Waldy for providing the image through PM and also for the tips.
If you have the same issue: Make sure to use Balena Etcher on Windows 11. I had to use a version of August 23 (1.8) to flash the image. With the newer versions I could not even choose the SD-card to flash it.
another very important task is to remove the conformal? coating inside the sd-card slot. The contacts are kind of covered with coating that prevents making contact with the card.
I used IPA-alcohol and a tiny brush. Please be very careful while doing so. I unfortunately bent a contact and then removed the aluminium-cover to bend the contact. The card now is (I hope) secured with hot glue.
anyway, the device is running again and I can finally read the intended book
