Change review author name to first and last letters only with asterisks between in WooCommerce

I am using "How to Change the Review Author Display Name in WooCommerce" so that I can change the review author shown on the site to be the first name followed by surname initial.

add_filter('get_comment_author', 'my_comment_author', 10, 1);
function my_comment_author( $author = '' ) {
    // Get the comment ID from WP_Query
    $comment = get_comment( $comment_ID );
    if (!empty($comment->comment_author) ) {
        if($comment->user_id > 0){
            $user=get_userdata($comment->user_id);
            $author=$user->first_name.' '.substr($user->last_name,0,1).'.'; // this is the actual line you want to change
        } else {
            $author = __('Anonymous');
        }
    } else {
        $author = $comment->comment_author;
    }
    return $author;
}

What I need to do is to use the first name only (not show the surname) and change all but the first and last characters to '*'.

So for example James becomes J***s and Michael becomes M*****l

Adjustments are:

• Display firstname
• James becomes J***s

comment added with explanation in the code

function my_comment_author( $author, $comment_id, $comment ) {  
    // NOT empty
    if ( $comment ) {
        // Get user id
        $user_id = $comment->user_id;

        // User id exists
        if( $user_id > 0 ) {
            // Get user data
            $user = get_userdata( $user_id );

            // User first name
            $user_first_name = $user->first_name;

            // Call function
            $author = replace_with_stars( $user_first_name );       
        } else {
            $author = __('Anonymous', 'woocommerce');
        }
    }

    return $author;
}
add_filter('get_comment_author', 'my_comment_author', 10, 3 );

function replace_with_stars( $str ) {
    // Returns the length of the given string.
    $len = strlen( $str );

    return substr( $str, 0, 1 ).str_repeat('*', $len - 2).substr( $str, $len - 1, 1 );
}