Wulfheart what's the error ?
Apart from created_at
not being a thing, I don't see any obvious issue with the payload. To backdate discussions and posts you could use my author edit extension which exposed new attributes for the author and dates to the API.
I don't think you can create API keys on FreeFlarum since you don't have access to the database so you might be a bit limited when using a user access token for the API.
Have you asked FreeFlarum if they can import the data for you? It would be much easier to migrate locally and then give FreeFlarum a data-only SQL dump to import, if they consent to doing it.