Quickly wanted to share my story with you, maybe someone can relate ;-)
I got my hands on a Fire HD8, Giza model (was advertised as Douglas, but oh well ...).
- Step 1 told me that I had to do the temporary brick, so I used the brick and continued with bootstep minimal
- The screen went black, "lsusb" did not show the device (neither did "dmesg"), tried to turn it off and on including pressing volume key -/+ simultaneously, and not pressing it - but it didn't work
- I tried one last time, and then I hoped it was still on and power would run off (I'm scared of opening the device ;-) )
- Two days later, I tried the bootstep minimal once again, but arrived at the same result
- I swapped the USB cable for another one, bootstep minimal found the device and I was presented with hacked fastboot mode, yay!!!
- I completed the fastboot step afterwards, got into TWRP, and then "adb sideload"ed LineageOS 15.1
So the process is very sensitive to the USB cable (although the first one had worked for the 1st step perfectly), and waiting for battery losing all its power is definitely an option, too.
Rortiz2, thank you very much again for your support on this device, and just a quick comment: the camera is not working (tried Open Camera - on Douglas, it is working fine there), but for me that is only a minor issue.