This is what I think is most likely as well. The capacity on the drive makes me think it’s a SSD and they can just spontaneously fail.

This is why you always need backups. It’s never a question of if, but rather when a drive will fail.

An inbound only DNS forwarding rule would be pointless. All DNS queries should be originating from within the network.

EDIT

I think I see what you’re getting at. Assuming that the firewall is running on the NAS vs on the router.

The OP doesn’t specify, but I would assume the firewall rule would be on the router, as that makes the most sense to force all DNS requests on the network to go through the pihole.

I agree.

So the solution, OP, is to set the DNS settings on your NAS to your router’s internal IP so the firewall can redirect the traffic to your new port.