Assuming that person trying to brute force your password knows that this passphrase generator exists and starts their search with all possible insult passphrases, otherwise they're searching in a much larger space

Of course, searching through all eight word combinations will be quite a bit harder. But that does not really protect you that much. If you are attacking passwords, you will try increasingly large sets of possible passwords. After you have gone through the million most common passwords and so on, you will also sooner than later spend a few minutes on trying all those insults before moving on to all eight word combinations, at least if this generator becomes popular enough to warrant inclusion in an attack.