Put "custom" option on security slider?
Hint: it's better to replace this checkbox with a new Security Slider position "Expert" above High, especially for mobile UX, which will have the same settings as High for the first time (so text to the right will be the same), and text will disappear when custom settings will have been made (and this position will preserve (!) custom settings while switching back and forth from other positions!)
I'm not sure about this approach, because it puts the custom option on the same "linear dimension" as the other security options. On the other hand, I agree it will perhaps make the semantics somewhat clearer.