Thank you for the help though I think this is not exactly what I needed. I searched a bit more and found a similar question to mine which had a very useful reply. NixVirt appears to be the optimal solution when you need to manually write XML configurations. While this approach works well, I need to read a bit more to see if configurations can be defined externally to my system flake configuration. i.e: when using build-vm, I want to use a separate flake file to define an entirely new NixOS VM system.
I also found information on the nixos-build-vms command, which can create several vms and connect them with just one declaration [1, 2]. This command is nice but still is not managed by a interface such as virt-manager.
Since the parameters of the result of build-vm are declared here, one could create a nix derivation to mimic this shell but generating a XML for virsh, but that’s a whole new challenge.
In the meantime, I think I’ll use the nixos-build-vms command for easily creating a network of VMs until I find a better approach with virt-manager and maybe NixVirt. If find one, I’ll comeback here.