And here we verge on Zen, because you can't deliberately pursue "a purpose that takes you outside yourself", in order to take yourself outside yourself. That's still all about you.

No, I mean, this looks reasonable. It probably gets at a higher truth of some sort. But it's the same mistake as thinking that a scientific hypothesis can't be true because it was inspired by an irrational belief. It shouldn't matter where you came up with your hypotheses--it only matters if it fits the evidence with the best explanation (and, ideally, makes the most falsifiable predictions that turn out to be true). Just because, for example, the drug company profits if everyone thinks their drug cures disease X doesn't mean that the drug doesn't actually cure disease X.

Just because my present self has an altruistic purpose only because my past self found pleasure in thinking that it would be an altruist in the future doesn't detract from my present self's actual altruism. (This is purely hypothetical, I am actually purely selfish across all time, space and possibility)