nVidia drivers for laptops on Linux are still garbage tier. I either have a choice of no hardware acceleration and working external displays on noveau (which toasts my CPU) or no external displays, but working hardware acceleration, on nvidia drivers which is useless for me.
I am definitely not buying an nVidia GPU ever again.
I have a friend who uses multiple displays on his 3080 using the Nvidia driver. They work for him.
It sounds like your issue is related to the dma buf issue that the kernel developers created. I thought that was fixed in recent versions of the open source Nvidia driver. What was the last driver you tried?
I am definitely not buying an nVidia GPU ever again.