From my experience with the pipeline so far and "sub-node" idea, I would say:
Keep pipeline controller with possibility to define where to run whole pipeline (same node/pod) Every step can be pushed to be executed on different pod Every step is a Task but step can consist of multiple function which are "sub-node" and they must be executed on the same pod/node where the functional_step is defined.
As a result if the pipeline requires sharing large files select the pipeline to run run on the same pod/node; if you want some steps to be executed on the others where docker container should be used– mark them;
UI appearance of "sub-nodes" per Task in the functional_step of the pipeline allow transparently see everything -> will be used more often, everyone likes pretty and clear UI 🙂