change-default-billing-country-for-non-existing-users-on-checkout-page.php

/**
 * Change the default country on the checkout for non-existing users only.
 */
function wb_change_default_checkout_country( $country ) {
	// If the user already exists, don't override country
	if ( WC()->customer->get_is_paying_customer() ) {
		return $country;
	}

	return 'DE'; // Override default to Germany (an example)
}
add_filter( 'default_checkout_billing_country', 'wb_change_default_checkout_country', 10, 1 );