I’d go more mentally competent. Are you capable of understanding the risks and what the end state actually is? You’ve had some time after the doctor has demonstrably given you the whole set of information to make an informed decision? You still want to move forward? K, here’s a surgery date.

Might just be me, but mental stability has some slippery connotations (thinking about the history of the term hysteria, different forms of social stigma turned into pathology, etc.) But we could very well be talking about the same thing using a different word.