Adding bulk posts with wp_insert_post() getting internal server error - php

Question

I have written a bulk import script where I import posts with the wp_insert_post() function using taxonomy names. Basically there are 4000 cities, and I want to add them as posts such as:
New York to London
New York to Amsterdam
New York to Berlin
etc...
But I'm getting an internal server error after adding a few thousand posts. I expect the total posts added to be 16,000,000‬ [4,000 * 4,000]
Here is the code:
<?php
require_once "wp-load.php";
ini_set("memory_limit",-1);
set_time_limit(0);
ignore_user_abort(true);
$term_query = new WP_Term_Query( array( 'taxonomy' => 'cities' ) );
if ( ! empty( $term_query->terms ) ) {
wp_defer_term_counting( true );
define( 'WP_IMPORTING', true );
foreach ( $term_query ->terms as $term ) {
$city1 = $term->name;
foreach ( $term_query ->terms as $inner_city ) {
$city2 = $inner_city->name;
$postTitle = $city1 . ' to ' . $city2;
$postSlug = $city1 . '-to-' . $city2;
$LowerPostSlug = strtolower($postSlug);
$post_args = array(
'post_title' => $postTitle,
'post_status' => 'publish',
'post_name' => $postSlug,
'post_author' => $user_ID,
'post_type' => 'conversion',
);
$found_post_title = get_page_by_title( $postTitle, OBJECT, 'city_post' );
$found_post_id = $found_post_title->ID;
if ( FALSE === get_post_status( $found_post_id ) ):
$returned_post_id = wp_insert_post( $post_args );
update_field( 'city_1', $city1, $returned_post_id ); # SET CUSTOM FIELD FOR city 1
update_field( 'city_2', $city2, $returned_post_id ); # SET CUSTOM FIELD FOR city 2
echo $postTitle . '</br>';
endif;
}
}
wp_defer_term_counting( false );
}