@Ascar Your idea here seems pretty much spot on, I don't really see any issues with it.
The only thing I would note is that personally for me, and especially in an actual prod environment, I recommend the NAS being it's own box running the OS on bare metal.
This doesn't mean there is anything wrong with what you are wanting to do, but for me, the device handling my backups and large amounts of data needs to be 100% functional, the last thing I want is something to go wrong with my hypervisor which somehow breaks my ability to get the backups to restore things.
There are plenty of people that do setups just like this though, especially in a homelab, and they usually don't run into issues.