Idk what to tell you. I linked to sources showing that flathub signs everything, and that flatpak refuses to install unsigned packages by default.
If you have anything contrary feel free to link it.
Also you multi replied to this comment. Sometimes I had this issue with eternity.
Proxmox is based on debian, with it's own virtualization packages and system services that do something very similar to what libvirt does.
Libvirr + virt manager also uses qemu kvm as it's underlying virtual machine software, meaning performance will be identical.
Although perhaps there will be a tiny difference due to libvirt's use of the more performant spice for graphics vs proxmox's novnc but it doesn't really matter.
The true minimal setup is to just use qemu kvm directly, but the virtual machine performance will be the same as libvirt, in exchange for a very small reduction in overhead.