I've been chasing this for two days now. I got it to work fine using a Nvidia gpu with legacy image and then i tried to use it on an 11th gen iGPU and proxmox completely hangs at the splash screen. If I remove the GPU passthrough it gets past that point and stops at LibreElect 11.0.1 text.
I've run out of ideas completely. I've tried all the recommended paths. I'm starting to think it just doesn't work with Iris Xe graphics