Disabled how? you mean you have your RX480 as main vga right?
Alright, that was how I was doing as well before this new two kexts. now I have RX480 as my main output and I don't need to switch between outputs anymore.I disable the IGPU in the UEFI. I had the RX 480 as my only display output but I kept getting random freezes. So I had to remove the kexts and re-enable the IGPU. Now I use the IGPU for the Clover boot screen and I have to manually switch the video input on my monitor to the RX 480. Which is a little annoying but not nearly as annoying as having to deal with random freezes.
Alright, that was how I was doing as well before this new two kexts. now I have RX480 as my main output and I don't need to switch between outputs anymore.
Maybe you should check your clover config file from the beginning, untick or disable anything unnecessary and try again.
Just wondering if the Lilu and Whatever kexts work in High Sierra on your RX480 build allowing you to boot with just that card. Also, any key changes you needed to make when moving to HS?
My Intel Pentium G3258 needed to use a different fake CPU ID to get through the updater but after installed I went back to i5 CPU ID. You may need to tweak a few settings but in general it is an easy upgrade.