Hi
I've just swapped a Sapphire RX580 for an XFX Radeon RX590 in my existing system: i7 10700K, Sonoma, AsRock Z490-M-ITX, OpenCore 0.9.6, SMBIOS iMac20,1
Everything seems to be working ok but the machine only reports 15MB (!) of RAM for the "Graphics" under About This Mac and if I'm honest...