Hey @Cassie, thank you for your answer.
Your solution calls reverse immediately so the nested timeline will play backward only.
I would like nested to play forward first, then to play backward.
I don't want to use a yoyo because I'll be adding animations between the two plays.
I have the feeling that once a timeline is added to a parent, you lose control over its playhead in favor of the parent's one.
Which means what I ask is impossible, unless I create an additional animation to manually implement the reversal on nested.