"Unless you adopt the (almost certainly nonsensical) position that everybody is entirely born, not made, you have to concede some degree of environmental influence on people's eventual properties(the degree of that influence is certainly a matter of debate; but almost certainly isn't zero)."
That is not a nonsensical position. Once you are born, your DNA does not change.
If you were not born a deep thinker, you will never naturally be a deep thinker, regardless of how much training or practice you have. You may be able to become good at deep thinking, possibly through practice or training, but you will never naturally be a deep thinker, since you were not born that way. That applies to everything in life. You are entirely born.
As for a person's ability to be able to use their personal abilities and skills to their fullest can be environmental and can be influenced by a number of factors including personal circumstances and upbringing. But again, a person who is born to take advantage of their situation will find a way to maximize their life and those who are not born that way will not, given identical circumstances.