Awesome, I am now able to edit my post! Thanks for your help!
There’s only one more issue, the key I am using is global. When I try with a key with write and read permissions only, I get: You are not permitted to view the requested resource. The API username or key is invalid.. When I edit the post via the GUI, there seems to be a PUT request to the topic’s URL (apart from the one that actually edits the post posts/post_id.json) which I can’t reproduce with a limited API key, only the global. I don’t see why, however, I wouldn’t be able to edit the post via the API even without this extra PUT request that happens in the GUI.
EDIT: Technically, my API key covers /t/:slug/:topic_id which is where the PUT request is pointing to.