It's possible your bios is not well configured.
If you want to try my bios settings. Import the CCTK files attached to this message with the dell command configure tool on Windows. I hope it's the last time you use windows on your computer ;-)
I also attach my bios configuration report if you don't have windows on your computer.
Also you can try:
-use another one USB port, if you machine(motherboard) has third party USB controller - use it
-boot without USBInjectAll.kext
-boot without usb fix in clover
-of course you flash drive is working and properly configured for install macOS
-use npci=0x3000 instead npci=0x2000
Are you still on High Sierra?
Please report here any news or changes if occurred...