I currently only use proxmox for VMs. Proxmox hosts a TrueNAS VM, TrueNAS controls all but the main (small) drive on the box, proxmox then has access to the other drives through TrueNAS. Kind of neat.
But I think it would indeed be simpler to only have TrueNAS and use it for both nas and VMs. I have no experience with TrueNAS' VMs.