The root cause of this issue seems to be the 'noeveau' driver not supporting all Nvidia cards.
While this could be worked around by first installing Debian and then the Proxmox packages I figured there must be a way to do this all from the ISO.

I finally cracked this thanks to a comment from u/Digger412 and figured I should make a full write up here.

1. When the ISO loads the menu option 'Install Proxmox VE' should be selected press 'e' to start editing the boot option.

2. On the 4th line (starts with linux) add nomodeset

3. Press Ctrl + x

4. After (attempting to) get DHCP the system will state 'Starting a root shell on tty3'

5. Wait for the system to detect the installation failure and drop to shell in TTY1 (Do not switch to TTY3)

6. Run chmod 1777 /tmp to unlock /tmp

7. Run apt update (I think this is partially to verify that /tmp is unlocked)

8. Run Xorg -configure To generate a new configuration file

9. Run mv /xorg.conf.new /etc/X11/xorg.conf to move the file to the config directory

10. Start editing the configuration file with your favorite editor.

11. Search for nouveau (there should be only one occurrence) and replace it with fbdev

12. Save and close the config file.

13. Run startx

14. Enjoy a working GUI installer

15. Use Ctrl + D to restart the system after the installer exits.

I hope this helps, feel free to share this info because this seems like a huge issue that requires way too much searching for a good solution.

PS: After reboot the screen seems to keep flickering but remote access should now work and it could probably be fixed by installing the proper drivers.

copy from => Proxmox Nvidia