Change email notification from “comment_author” to “WP username”?

I have an issue where I have a site using MemberPress. When a MemberPress user subscribes, they get a WP username and nickname. Unfortunately that nickname is, by default, their first and last name, which is what’s used when called by the “comment_author” shortcode in the Notifications settings.

That means when notified of a comment follow-up, the “comment author” listed in the email is their full legal name, and not user name. Not ideal!

Can I modify the template with something other than [comment_author] that would use their WP username (if they have one)? Or is there a way to do that with a function?