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.