I have installed proxmox and generally played with it, Here are some of what i find worth to be mentioned:
Importing distro is by just wget .iso URL to proxmox import path, Then it will be readable to proxmox. (Verification of the .iso should be done manually asaik)
Proxmox used by many big hosting companies, having whonix compatible with it will give whonix easy installation for those servers
Proxmox is debian plus kvm which whonix have expert with both of them not something new/strange.
Big data hosting (cluster like) can be easily as well served through this design which is already used for these type of things. (Whonix currently doesnt support clustering, But this just make it easier to adopt)
Proxmox is debian+kvm by default from upstream we dont need to do any effort to have them both together and working/linked.
Almost we can say it is qubes like distro.
Proxmox not really free in price, yes the free version is usable but it has subscription method in which there are upgrades you cant have inside the free version (there is repository for proxmox wont work without subscription)
Proxmox main OS is actually cli only, If you want to view it into graphical mode the way to do that by using URL in your browser (can be it modified? but still bad compared to qubes,debian…hosts)
Proxmox doesnt accept (seems to be) preconfigured libvirt xml images, It only accept .iso formats (Even if it accept it, Its not piece of cake like just importing the images) ← Couldnt find sources to prove my conclusion wrong about this point.
Whonix for its operation needs at least limited control of the host operating system. Whonix VirtualBox ova’s and KVM libvirt XML files instruct the host virtualizer how Whonix-Gateway and Whonix-Workstation need to be configured. Most importantly, Whonix-Gateway having two network interfaces, external and internal as well as Whonix-Workstation having only one internal network interface. Whonix-Workstation internal network interface connected to Whonix-Gateway internal network interface. Plus other settings (see source code) but keeping it simple.
We don’t have Whonix-Gateway.iso, Whonix-Workstation.iso. And even if we had, these could not configure the required host virtualizer settings.