Im just the beginner with proxmox/linux stuff so go easy on me if I say something stupid
I had problem installing Proxmox on my computer too, old Alienware Alpha with GeForce GTX 860M. After a lot of research and trying diffrent things finally steps from this website helped:
https://robertoviola.cloud/2020/04/16/proxmox-no-screen-during-installation/
Because I have 4k TV connected to this computer I had to change
to 4k resolution and it work, I was able to finish installation process.
Maybe this will help you.