It is not clear, did you successfully unlock your bootloader?? If yes, did you flash the right phh treble "*.img"?Yes you have reason when you wrote your post because I had use DC-unlocker for my bootloader but when I installed test point, my tablet has turned off and no longer turns on, no longer charges.
I tried power bottom during 30 s, volume bottom etc.
My tablet is dead.
I read that that sw is able to downgrade emui, contact the dc-unlocker team or you could restore the original firmware with the Huawei suite.
I suggest to search on the web, any android devices should boot in fastbost or recovery mode. Do you try to downgrade the firmware or flash a wrong one?
But android 8 booted? i think no. There is a guide to downgrade the os, you cannot simply flash an older firmware.