Change the display name to default to just the first name using the Force First Last WordPress plugin.

my_ffl_display_name_just_first.php

<?php
/*
 * Change the display name to default to just the first name using the Force First Last WordPress plugin.
 * https://wordpress.org/plugins/force-first-last/
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 */
function my_ffl_display_name_just_first( $display_name_order, $first_name, $last_name ) {
	$display_name_order = array( $first_name ,'' );
	return $display_name_order;
}
add_filter( 'ffl_display_name_order', 'my_ffl_display_name_just_first', 15, 3 );