Their use cases are a bit different, no? Proxmox is a general hypervisor. You can run whatever you want on it. NAS is one workload that could be run on top of Proxmox. TrueNAS is a NAS first solution, hypervisor second. And that's the overlap with Proxmox. You could think of your core use case:
Do you want mainly a NAS that can run a few services too?