I *think* a unix timestamp will do the trick (it's a unique long string of numbers). Just have to add a column to the 'orders' table and figure out how to modify the script that posts to it, then call that column for the Order ID in the customer confirmation email.
UPDATEMaking some progress!
I added the column 'unix' to the 'orders' table in MySQL and set it to bigint(32)
Then I modified the order.php page [storefront/model/checkout/order.php] and inserted the following code at line 215
unix = NOW(),
On checkout, this inserts a numeric string like 20180705230844 in the 'unix' column.
Now working on how to modify the order_confirm.tpl to use the 'unix' column instead of the 'order_id' in the email send.
UPDATE #2Got it!
Changed line 409 of order.php to
$subject = sprintf($language->get('text_subject'), $order_row['store_name'], $order_row['unix']);
Inserted to order.php on line 451
$this->data['mail_template_data']['unix'] = $order_row['unix'];
Changed line 29 of order_confirm.tpl
<td class="align_left"><?php echo $text_order_id; ?> <?php echo $unix; ?><br/>
UPDATE #3Changed line 409 to just plain text
$subject = sprintf('Receipt for Your Payment to Example');
Changed line 29 of order_confirm.tpl
<td class="align_left"><?php echo 'Order Number:' ?> <?php echo $unix; ?><br/>
Also timezone was wrong, so added to the top of php.ini
date.timezone = "America/Los_Angeles";
UPDATE #4Although timestamp is a unique number, I wanted the Order Number in the email send for the customer to be more random in it's appearance, and also shorter in length.
My solution was to simply take the timestamp and subtract year,month,day:
Changed line 215 in order.php:
unix = NOW()-201807050000,
Ex.
Order Number: 20115930