WordPress Related Posts by Title and Category

Probably the simplest approach is to combine the category query with a search query (untested):

$cat_query = new WP_Query( array(
    'cat'            => $cat_id,
    'post__not_in'   => array( get_the_ID() ),
    'no_found_rows'  => false,
    'posts_per_page' => 6,
) );

$title_query = new WP_Query( array(
    'post__not_in'   => array( get_the_ID() ),
    's'              => get_the_title(),
    'no_found_rows'  => false,
    'posts_per_page' => 6,
) );

$related_posts = array_merge( $cat_query->posts, $title_query->posts );

// Need to sort the posts so they're not grouped.
$dates = array_column( $related_posts, 'post_date' );
$related_posts = array_multisort( $related_posts, $dates, SORT_NATURAL );

// Limit to six posts.
$related_posts = array_slice( $related_posts, 0, 6 );