Adb devices doesnt help you with fastboot and USB driver neither…
So read this when you use windows
You put your phone into fastboot/bootloader mode by turning it off and than restarting by homding power+vol-down
This advise would not prevent a brick and can rather cause exactly this (although more important for the FP4) best is to lock before booting into the system, however not possible with the FP script. Thus the most important thing is, only flash a version with a newer security patch level, to be as safe as possible see @Urs post