I was going to spend this evening working on browser extension, but I think it would be better spent providing a /data export utility for fellow ex users.

From my initial researches, it looks like /u/username.json is the best bet, as it gives a JSON array of all posts written by username, along with like and comment data. It accepts a max_time=timestamp query param, and a _ query param, the function of which I am not sure of.

To iterate through all the pages of posts from a certain user, start with their profile URL w/ .json tacked on the end, fetch all the items, get the datetime of the last item, convert that to a timestamp, fetch the same URL with ?max_time=timestamp, repeat until an empty array is returned.