HI @ashfire ,

After second read of documentation I think the endpoint you choose should work (with scope="all-apis"). According to docs it will let you call both REST API , so sorry for confusion in previous message.

The reason it works out of the box for workspace administrator is because workspace admin has all necessary permission in workspace. 
But for any other user I think you need to grant them explicitly. Could you check if your Service Principal has appropriate permission on your model serving endpoint?

Manage model serving endpoints | Databricks on AWS

Also, please check if your Service Principal has been added to the workspace:

And lastly, check if he has workspace access entitelment enabled: