On disadvantages first bullet point. Proxmox is free. The no-subscription repository has all needed updates for a fully functioning system. The lack of subscription only indicates the lack of a support contract and is reflected in logs when attempting to obtain support.
It’s meant to be headless, so CLI-only with a web interface is quite workable.
I would add to this thread that Proxmox has excellent support for something Qubes does NOT support out of the box, that’s LXC containers, plus all the pre-built appliances already posted on Turnkey Linux. It is my suggestion who have Whonix also contribute to this library so that it might be one of the default templates there for easy import and setup into proxmox as an LXC container, which has native hardware like speed and a very low resource footprint.