bk-wc-checkout-thankyou-title.php

<?php
function bk_title_order_received( $title, $id ) {
	if ( is_order_received_page() && get_the_ID() === $id ) {
		$title = "Thank you for your order!";
	}

	return $title;
}

add_filter( 'the_title', 'bk_title_order_received', 10, 2 );

Thank you this works perfectly!