You can use Ecto embedded changesets for this, simply define all your CRUD changesets and validation in Ecto as you normally would.
Your contexts decide what to do with those changesets, how to read, create, update, and delete, just follow the same pattern of changeset validation, and pipe to an external API instead of Ecto.Repo. Remember that Ecto.Repo interfaces with an external system too as it maintains a connection pool to an external database.
You will probably want to think about a connection pool for the API. Here is a tutorial of following the similar model that Ecto uses for maintaining a pool using poolboy.
If you are using a http interface then req which is built on top of finch/mint/nimblepool also has pools integrated, the nice part being finch/mint/nimblepool is already included with phoenix.
If you are using the API at a rudimentary level with your own http request handling I would create a thin abstraction around around it to handle auth token refresh (something req can help with too with a retry error step) and mapping errors to changeset field errors and so on and this will make it easier to test also.
