Try this revised EFI, hopefully it will work with your AMD 6-core setup.
The error was caused by the Kernel >Quirk > SecureBootModel being set to Default, when it needs to be set to Disabled for High Sierra.
I changed the PlatformInfo to MacPro6,1, as that is probably a better match with your old Radeon HD 6670 dGPU. iMac14,2 expects an IGPU to be present, which no AMD systems have.
Just rename the folder to EFI and it should be good to go.
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