Large WooCommerce query throws fatal memory errors - wordpress

Question

I have a site that has tens of thousands of orders, which I need to compare the billing and customer/user emails and show a flag if they don't match. One of the stipulations is that I'm unable to add any metadata to the orders. So my solution is to just add a custom column, and compare the emails on the fly when the orders list is rendered. That works just fine.
add_filter( 'manage_edit-shop_order_columns', 'mismatched_orders_column' );
function mismatched_orders_column( $columns ) {
$columns['mismatched'] = 'Mismatched';
return $columns;
}
add_action( 'manage_shop_order_posts_custom_column', 'mismatched_orders_column_data' );
function mismatched_orders_column_data( $column ) {
global $post;
if ( 'mismatched' === $column ) {
$order = new WC_Order( $post->ID );
$customer = $order->get_user();
$result = '';
$billing_email = strtolower ( $order->get_billing_email() );
$customer_email = '';
if ($customer) $customer_email = strtolower ( $customer->user_email );
if ( $customer && ( $billing_email != $customer_email ) ) {
$result = '<span class="mismatched-order" title="Possible order mismatch">Yes</span>';
}
echo $result;
}
}
My issue is when trying to add sorting. Because I'm not accessing any post metadata, I don't have any easy data to sort via the main query. My solution here was originally to hook into pre_get_posts, grab all the orders in a new WP_Query, then loop through them and add the ones that had mismatched emails to an array for use in post__in.
This works/worked fine on my small dev site, but throws fatal memory errors when trying to loop over any more than about 8 or 9 thousand posts (out of a total of 30-40 thousand). Increasing memory isn't really an option.
add_filter( 'manage_edit-shop_order_sortable_columns', 'mismatched_orders_column_sortable');
function mismatched_orders_column_sortable( $columns ) {
$columns['mismatched'] = 'mismatched';
return $columns;
}
add_action( 'pre_get_posts', 'mismatched_emails_posts_orderby' );
function mismatched_emails_posts_orderby( $query ) {
if( ! is_admin() || ! $query->is_main_query() ) {
return;
}
//Remove the pre_get_posts hook so we don't get stuck in a loop
add_action( 'pre_get_posts', 'mismatched_emails_posts_orderby' );
//Make sure we're only looking at our custom column
if ( 'mismatched' === $query->get( 'orderby') ) {
//Set our initial array for 'post__in'
$mismatched = array();
$orders_list = get_posts(array(
'post_type' => 'shop_order',
'posts_per_page' => -1,
'post_status' => 'any',
'fields' => 'ids'
));
//And here is our problem
foreach( $orders_list as $order_post ) :
//Get our order and customer/user object
$order_object = new WC_Order( $order_post );
$customer = $order_object->get_user();
//Check that billing and customer emails don't match, and also that we're not dealing with a guest order
if ( ( $order_object->get_billing_email() != $customer->user_email ) && $order_object->get_user() != false ) {
$mismatched[] = $order_post;
}
endforeach; wp_reset_postdata();
$query->set( 'post__in', $mismatched );
}
}
I would seriously appreciate any insight into how I could either reduce the expense of the query I'm trying to run, or an alternate approach. Again, just for clarification, adding metadata to the orders isn't an option.
Thanks!