We have our own purchase ordering system that integrates with Shopify. You set up restock level for each product and when stock follows below this it gets added to a report each day. From here you can generate purchase orders to your suppliers that get emailed.
We specialize in bespoke and custom solutions so we add features for our clients when items are missing from our core application.
If you would like to test it out please get in touch.
With tools like that you would either control things like minimum-inventory as simple metafields on the productVariant itself or in the config of such a script.
This approach works well instead of a dedicated niche app if you also have other automations needing implementation.
Things like this can also be done with services like zapier,integromat,airtable wired to google-sheets/docs; useful if your already using those services.