WordPress query: merge meta key (number) values and sort

There’s a lot of great documentation on multiple column sorting using orderby however I can’t seem to find a good WP_Query solution to this problem.

Each post has two meta keys: rating_au and rating_overall.

I want to query posts by rating_au as a priority and then falling back to rating_overall if no value for rating_au exists.

Example Table

Example Output

I have got as far as the following query:

I think I’m probably misunderstanding how this translates to MySQL but it does not produce the desired outcome (it simply lists descending order for rating_overall).

I’m wracking my brains on this and am looking to avoid $wpdb or running multiple queries and merging values.

It’s likely there’s an entirely different approach to this problem. Any suggestions are gratefully received!