I recommend the solution provided above. If you don’t like it for some reason, you can use GitHub - pfaffman/discourse-allow-pm-to-staff: Allow private messages to be sent to staff for users who could otherwise not send private messages. · GitHub.
I’m going to encourage my client who’s using it to stop, but it was just updated to deal with some changed system setting names and I believe (but do not promise) that it’s reasonably unlikely to break any time soon.