As for x-vga=on I don't know if it's needed or not, I think @nikitas got it working without it too but if it's needed maybe something is using that on the host. If you still see anything on screen from the host then the GPU is not isolated completely. The host should not use the GPU at all which might not be simple to do on a laptop. On a desktop machine you'd pass through a secondary card while the host is using either the embedded graphics or another card. Also you may need to check vfio groups and if the GPU can be passed on its own or there are other devices in the same group that may need to be passed through as well. I think you should check docs on vfio pass through and test with a Linux guest to see if it works before trying AmigaOS.