Add thumbnail to Next Post/ Previous post on Astra Theme

functions.php

<?php

/**
 * Add thumbnail to Next Post/ Previous post.
 *
 * @param array $args Arguments for next post / previous post links.
 * @return array
 */
function astra_change_next_prev_text( $args ) {
	$next_post = get_next_post();
	$prev_post = get_previous_post();

	$next_text = false;

	if ( $next_post ) {
		$next_text = sprintf(
			'%s <span class="ast-right-arrow">→</span> %s',
			$next_post->post_title,
			get_the_post_thumbnail( $next_post, 'small' )
		);
	}

	$prev_text = false;

	if ( $prev_post ) {
		$prev_text = sprintf(
			'<span class="ast-left-arrow">←</span> %s %s',
			$prev_post->post_title,
			get_the_post_thumbnail( $prev_post, 'small' ),
		);
	}

	$args['next_text'] = $next_text;
	$args['prev_text'] = $prev_text;

	return $args;
}

add_filter( 'astra_single_post_navigation', 'astra_change_next_prev_text' );