I had to go back to HighSierra after installing Mojave, because: the video driver does not work (jittery screen and dual monitors are not supported). Will try Mojave again when the official Nvidia web driver is released
The method outlined in the op worked for me but I had already gotten the information from another web page Z68x Mojave install it looks to be very similar method if not exact.
I had a HD 6670 installed and it was rendered useless in Mojave (as explained elsewhere), but RX 560 works a peach.
Also DVI port does not work. I use the HDMI port out --> tv and display port --> Dvi on my monitor.
At first my firewire Pci card was not recognised in Mojave. I have an external firewire sound card. Only the onboard sound was recognised, then mysteriously after a few days the firewire sound option appeared out of nowhere. This is the best upgrade so far.