I'm running Monterey on a 4,1 flashed into 5,1 with exact same GFX card as OP. It works perfectly. With OCLP, I get boot screen (including the one to enter FileVault password). Zero issue but it prolly as far as 4,1/5,1 can go.The 5,1 has an issue with its early CPUs, that prevent it properly running beyond Monterey.