All partitions wiped, USB Debugging was not enabled, ADB and fastboot can not see Nexus 5
@Hellreaver
Actually: Not yet a brick.
I managed to install a custom ROM along the lines explained here:
https://www.youtube.com/watch?v=6m5pJSYm19I
Flashing from the USB stick was still possible, since I had TWRP 3.3.1-0 already installed on my Samsung S6 G920F.