I think the problem is that a 'fluent' chain of calls already expresses a sequence, so the way that 'after' resets the context to start a new branch feels very awkward ... like a GOTO or something

It's telling that the example relies on arbitrary indentation (which a linter will get rid of) to have some hope of comprehending it

Possibly this was all motivated by a desire to avoid nested structures above all?

But for a branching graph a nested structure is more natural. It'd also probably be nicer if the methods were on the task nodes instead of on the workflow, then you could avoid the 'step'/'then' distinction and have something like:

e.g.

    testWorkflow(
        llm
        .then(decider)
        .then(
            agentOne.then(workflow),
            agentTwo.then(workflow),
        )
    )