1) Make sure you have WhateverGreen.kext, or put it in your KEXT folder on your EFI volume.
2) Using Clover Configurator, if you have a discrete GPU (AMD/NVIDIA), add this bootflag: shikigva=32. If you have an integrated GPU you're using, the flag is shikigva=40.
3) Also add this bootflag to make it think you're using a different motherboard than the Mac Pro that is the apparent issue: shiki-id=Mac-7BA5B2D9E42DDD94
4) Restart.
Try that. As a side-note, make sure you have the latest USB patches, they changed for 10.14.5 from 10.14.4.