I mean, if you have a VPN server setup in the LAN, your NAS is already internet accessible.
I think system-level wireguard is a good idea because normally the NAS is the last piece of equipment gets offline (except router of course) .
Not necessarily. it is not "directly" internet accessible, as in your NAS itself, isn't publishing itself as available on the public internet on a public IP via NAT Rules for the world to scan every day and discover (and potentially reveal info of what it is TrueNAS OS or something else..)
PFSense is designed around security as a perimeter device, TrueNAS, not likely as much, it is a NAS OS at its core.
If you run a VPN Server with in your network on another VM or system, sure, it could be compromised and someone now can get access to your internal LAN, but, that does not mean they can get into your NAS, if it is set up properly and secured.
Security is all about layers and segmentation to make it harder to be compromised, for me, putting your NAS, even if open to only 1 port, allows someone to find a potential exploit with in the implementation of OpenVPN and get into the OS layer, which can cause far more damage and say good by to your entire NAS possibly.
Sounds extreme, but it is not a matter of if, but when for all of this lovely tech we rely on.