Fun facts:
* They switch around button colors to trick you into undoing your actions
* They are very eager to log you back in if you, for example, try to access help. This reactivates your account.
* You cannot delete your account on Messenger. They removed the option. Deleting your Facebook profile does not delete your messenger account
* Their doc for deleting your messenger account lies/is out of date.
* There is no shortage of dark patterns in their account delete process.
I ended up manually deleting all my content, unliking everything, and deleting all my posts.
More fun facts: they don't show you all of your content. If you go back and try to clean up your account, do it multiple times. Keep doing it until Facebook can't find anything more to show you. I assume this is a technical optimization, but it means that there is no reliable to see everything you've posted (from the web UI).