SPICE

You can access the full 4k GUIs of Proxmox VMs via SPICE. It is already baked into proxmox.

On the client, install SPICE support:

sudo apt install virt-viewer

Then in proxmox, request a SPICE session via the console dropdown. Double-click the downloaded certificate to start a SPICE session.

WARNING: it is only valid for a moment, so make sure you click the download shortly after it arrives.

NOTE that the actual display driver to use is NOT SPICE (which crashes). Instead, for the proxmox VM Display, use VirtIO-GPU. I used 80GB memory.