Root Fire 7 (9th) OS 7.3.1.7? Push my limits

Hello dear community,

I have a 9th generation Fire 7 with OS 7.3.1.7 from a good friend.
The device is to be bricked.
Professionally I am an IT system electronics technician and as a hobby I am a tecnican-freak, but this is where I reach my limits.

I have already opened the device and tried the hardware method, but I also get an error message in the terminal. So that I cannot unlock the device or the bootloader.
With MTK-SU this is no longer possible with the OS as I have already read and...

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