Just sending over a request every 500ms lacks some sanity checks to make sure you really don’t exceed rates or run into other resource limit throttles.
Some notes here:
https://help.shopify.com/en/api/reference/rest-admin-api-rate-limits#recommendations
Since you talked about creating Orders and you’ve shared the error response I’ll make the bold assumption that this store is an affiliate one - so a dev shop.
Hit up the docs for order create:
https://help.shopify.com/en/api/reference/orders/order#create-2019-04
There’s a note in there:
If you’re using this endpoint with a trial or Partner development store, then you can create no more than 5 new orders per minute.
So based on your current plan to throw over an order every 500ms you’ll hit that quickly.