Fantastic work, Renate
I managed to get adb working! I also attempted to follow the same instructions you gave DogilD by placing the update.zip file on an SD card, running start autoupdate and plugging it in. I assumed this was the next step, correct me if I'm to trigger-happy.
Unfortunately, my display doesn't give any valuable information during the flashing phase, my screen just looks like this:
https://we.tl/t-vT7pHa8P0v
It does this for a few seconds before rebooting to the Onyx logo, the same as before.