We use Primal's relay for dev testing, but you'd probably do well using any of the primary public relays.

NWC payloads are E2EE so privacy is pretty excellent. NWC events do identify themselves as NWC events though, and afaik for the life of a NWC string, it has a stable "identity" from the relay's perspective. The only other thing you might need to be concerned about is IP addresses.

The NIP is a pretty easy read imo: https://github.com/nostr-protocol/nips/blob/master/47.md