Hi,
I successfully updated my z68x build with i7 2600k cpu and rx560 from 10.13.6 to 10.14. The fcpx freezing reemerged and after spending A LOT of time on the forum here I finally nailed it. I think.
I enabled iGPU in bios, I have inject Intel enabled, lilu and weg kexts in efi/clover/kext/other and config.plist as per
https://www.tonymacx86.com/threads/guide-intel-framebuffer-patching-using-whatevergreen.256490/ And the most important thing is changing to Mac Pro 5,1. See here
https://support.apple.com/en-us/HT208898
No freezes with 2 monitors in DP and HDMI in rx560.
Oh, and the hd3000 shows in sys report but is obviously not utilised in Mojave.
Try it and hope it helps.