Getting featured image from WP_Query

Suggest to reformat the data like such (untested):

function load_more_blogs() {
    ...

    $results = array();

    foreach ( $query->posts as $post ) {
        $results[] = array(
            'post'  => $post,
            'image' => get_the_post_thumbnail( $post->ID, 'post-thumbnail' ),
        );
    }

    wp_send_json_success( $results );
    exit();
}

This will give you an array of arrays, with the inner array have two entries: post and image. They can be accessed like such:

print_r( $results[0]['post'] );
print_r( $results[0]['image'] );