I'd like to get the RX560 working, but if I can't then integrated graphics is acceptable. The EFI folder I just posted is intended for use with the RX560 GPU.
I was under the impression that "InjectIntel", "InjectATI", and "InjectNvidia" were all deprecated and no longer used, which is why I left them all false. And I misunderstood the use case for ig-platform-id.
Given that I want to use the AMD GPU, should I then set "InjectATI" and change the value of ig-platform-id to 3E920003 (or rather 0300923e when byte-reversed for use in config.plist)?
I believe InjectATI and InjectNvidia are for really old cards, but InjectIntel still seems to be needed for integrated graphics.
However, for the RX 560, I think all the graphics injects should be false since you don't want active Intel graphics (you just want the integrated GPU there as a compute device -- so it should be enabled in BIOS but configured as headless in config.plist). The ig-platform-id is in regular order if listed as a string in the graphics section, or byte-reversed if encoded as data in the device properties section. I think putting it in the graphics section is the traditional way; putting it in device properties is as I understand it more forward-looking.
Anyway, for my Coffee Lake RX 580 build I used SMBIOS iMac18,3 (which has a GPU) and ig-platform-id 0x3E920003 as you say, and it looks like I also set a device-id -- here's what that section looks like in CC, and the whole config.plist is attached (but without the unique IDs, so if you want to reuse it you'd have to go to the SMBIOS screen and hit both generate new buttons).
Finally, I don't remember if you mentioned the brand of RX 560, but I hear XFX ones typically need some additional patches or a firmware reflash or something.