I don't want to come across as contentious but will. you. please. just. use. a. real. database.
A flat-file architecture only scales so far. This is probably the only forum on the internet where a thread passing 500 comments is an actual problem - and remember, you're planning on getting rid of pagination, which means no theoretical limiter on request sizes. In a few years when 500+ comments threads become the norm among another million users, disk I/O is going to be more and more of an issue.
Racket has a perfectly good database library, just kick the problem upstairs and be done with it.
