Example PHP of how to enable sorting by term name. `orderby=term_name` and `orderby_tax={taxonomy_name}`

sort-by-term-name.php

<?php
/**
 * Filter EP args to add support for orderby term_name
 *
 * @param array $formatted_args
 * @param array $args
 */
function ep_add_term_name_sort_support( $formatted_args, $args ) {

	// Handle meta_value / meta_value_num sort
	if ( ! empty( $args['orderby_tax'] ) && 'term_name' === $args['orderby'] ) {
		$order = 'desc';

		if ( ! empty( $args['order'] ) ) {
			$order = strtolower( $args['order'] );
		}

		$formatted_args['sort'] = array(
			array(
				'terms.' . $args['orderby_tax'] . '.name' => array(
					'order' => $order,
				),
			),
		);
	}

	return $formatted_args;

}
add_filter( 'ep_formatted_args', 'ep_add_term_name_sort_support', 10, 2 );