That symptom usually means the RX 6600 doesn’t get proper support in Sonoma by default. Monterey still included working drivers for Navi23 GPUs, but Apple trimmed support in Ventura/Sonoma. Two things to check:
Bootloader patches: If you use OpenCore, enable agdpmod=pikera boot arg for AMD Navi cards.
Kexts: Make sure WhateverGreen.kext and Lilu.kext are the latest versions, otherwise the GPU won’t initialize.
SMBIOS: Use a model known to support AMD dGPUs (e.g., iMac20,1 or MacPro7,1).
Logs: Boot with -v and see at which point the handoff to the GPU fails.
If you want full Sonoma compatibility, confirm that the current WhateverGreen release supports RX 6600. Without it, you’ll get exactly the black screen after the logo.
