> we don’t want to write one particular arrangement of concurrent operations, but rather want to create a meta system that lets us string together arbitrary ones. Would you agree that the actor abstraction becomes useful again for such cases?

Actors are still just too general and uncontrolled, unless you absolutely can't express the thing you want to any other way. Based on your description, have you looked at iterate-style abstractions and/or something like Haskell's Conduit? In my experience those are powerful enough to express anything you want to (including, critically, being able to write a "middle piece of a pipeline" as a reusable value), but still controlled and safe in a way that actor-based systems aren't.