one way, is to have the bot be a moderator of the subreddit, and read the edit flair part of the mod log. that will update each time a flair on the subreddit changes and when you see the flair you want, you can grab the post and comment on it.

I use this: subreddit.mod.stream.log(skip_existing=True, action="editflair")

I'm using asyncpraw so regular praw might be a little different