Suppose someone chooses to use a seed (mnemonic) that is easy to memorise, like
true kid true kid true kid true kid true kid true kid - using a passphrase, of course.
The benefit is clear: he'd only have to memorise 2 words and the passphrase (and he already memorises his current passphrase). If the person forgets what were the 2 words, he can still easily brute force it, provided he remembers the passphrase and that there were 2 alternating words.
Now, how unsafe/safe is that? I can't really imagine there's someone trying to brute force passphrases of simplistic seeds out of nowhere. If my math is right, 2 alternating words mean 2048 * 2047 * 3 possible combinations (1st word, 2nd word and 12/18/24 length).
Considering a relatively short 8 alphanumeric character passphrase, the number of possibilities is 2048 * 2047 * 3 * 368 = 3,54 × 1019, which is a huge number. For comparison, it cost this guy ~425 USD and 30 hours to brute force 1 trillion seeds, and not only he knew there was a prize, but also had an address to look for.
Are the any flaws in these thoughts? Any pattern in the seed inner workings that could shorten the brute force job?
Edit: if you can, please specify a theoretical scenario in which an attacker could attempt to steal funds.