assume a meek person becomes a very good public speaker and as a result becomes more assertive in all aspects of life. Are they still a meek person

This somewhat applies to me (though I'm a mediocre public speaker at best, but I'm no longer intimidated by public speaking like I was when I was younger). For me I find the confidence/assertiveness is localized: I can be confident/assertive in familiar environments, but if I'm in an unfamiliar environment I tend to revert back to my normal self.

Then when I'm in an environment where confidence/assertiveness is required, it does tend to wear me out over time, because it's an aspect of my personality I have to "turn on".

Many years ago I knew someone who I thought was quite extroverted but admitted he too was introverted by nature and that it was something he had to "turn on". He was better at it than I am, but I think it still tended to wear him out the same way (though perhaps not as quickly as it wears me out).