Change “Display name publicly as” to not use email address

I have used the code snippet from automatically-generate-an-account-username-changed-to-name/ to ensure usernames are created with their email addresses.

The problem is that it also makes the “Display name publicly as” the email address.

When the user posts a product review, it shows their email address, which is privacy issue.

1. How can make the “display name publicly as” with just the start of the email before the @ symbol?
2. My preferred solution would be to ensure the First Name and Last name from the billing details is captured for the automatic registration of at checkout (using the “create account?” checkbox”). And then use this to generate the “Display name publicly as” value so that their product reviews show “first name last name”

Thank you for your help in advance.

The page I need help with: [log in to see the link]