How to know when an order is ready to ship (stock is available) without checking every single item?

Sadly, no, in theory Shopify Plus could allow you to script something to check all the orders or Arigato could have a workflow, though it’s complicated enough that we haven’t been able to make one work yet.
We have Arigato add a tag to orders with a back-ordered part# and supplier name so that we can at least see the part number in the open orders view, that helps a bit.