Delete all Guest Posts?

I have a user who signed up using his real name, requested his account be deleted for privacy reasons, and even after deletion, his content still exists under his real name, just that now he's a "guest". Needless to say, he's throwing a hissy fit because being a "guest" does nothing for his privacy as his username is still his real name. How do I change his username, especially now after his account has been deleted?
 
I have just fallen into this ridiculous trap and now find myself with 7000 posts to delete manually.

Please tell me there's a better way to do this in 2021?
 
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