Hum I don´t have access to one of the newer AMD Cards so no way to test it but maybe the build in ATIInjection in OZ may not handle them correctly. Have you tried to inject the Cards properties directly via DSDT instead of using OZ Build in Injection? Also did you check if the Cards Device and Vendor ID is even present in AMD9000Controllers Info.plist?
I use only ssdt for CPU and IGPU, didnt tryed to add dsdt with AMD gpu. When i look at AMD95100 kext, there i can find rx460 id.
Other problem, that in my config almost everything worked OOB, exept new GPU, so i kinda forget a lot about DSDT variants of making.
I would be glad, if you share instructions what should i do.