Unless I'm misreading, you can only create a custom weakness if the creature lacks an existing weakness equal to at least 2 + half your level.
It's probably better to always set an existing weakness higher when you can than a custom weakness that only you can exploit. But the whole flavor of the custom weakness is so fun, I'd like to be able to apply it even if it's a suboptimal choice because there's a thematic choice in jeopardy.
Any thoughts on making the custom weakness an option even if there is a weakness you can set?