Personally, I think Proxmox is somewhat unsecure too.

Proxmox is unique from other projects, in it’s much more hacky, and much of the stack is custom rather than standards. Like for example: For networking, they maintain a fork of the Linux’s older networking stack, called ifupdown2, whereas similar projects, like openstack, or Incus, use either the standard Linux kernel networking, or a project called openvswitch.

I think Proxmox is definitely secure enough, but I don’t know if I would really trust it for higher value usecases due to some of their stack being custom, rather than standard and mantained by the wider community.

If I end up wanting to run Proxmox, I’ll install Debian, distro-morph it to Kicksecure

If you’re interested in deploying a hypervisor on top of an existing operating system, I recommend looking into Incus or Openstack. They have packages/deployments than can be done on Debian or Red Hat distros, and I would argue that they are designed in a more secure manner (since they include multi tenancy) than Proxmox. In addition to that, they also use standard tooling for networking, like both can use Linux Bridge (in-kernel networking) for networking operations.

I would trust Openstack the most when it comes to security, because it is designed to be used as a public cloud, like having your own AWS, and it is deployed with components publicly accessible in the real world.