I agree with this.
I think one could address that simply by tweak the quoted sentences to “It has been determined that it’s most likely a good idea to start object-level work on a specific cause/intervention (e.g., starting a charity, starting a new programme in an existing org or government, doing independent research and advising key decision-makers). Now the hard work begins of actually doing that.”
(I’m also not sure the implementation stage will always or typically be harder than the other three stages. I don’t specifically believe the opposite; I just feel unsure, and imagine it varies.)
One minor quibble with your comment: I think “do an Econ PhD [and during this or afterwards try to estimate the philanthropic discount rate]” should probably not by itself be called “implementation”. It’s more object-level than doing prioritisation research to inform whether someone should do that, but by itself it doesn’t yet connect to any “directly important” decisions. So I’d want to make some mention of later communicating findings to key decision-makers.
[Btw, thanks for this post, Nuño—I found it clear and useful, and liked the diagram.]
That’s all fair. I would endorse that rewording (and potential change of approach)