these aren't really traits you can just "decide" to improve

True! Instead, these are skills that you can train. "Just decide to be extroverted" will work about as well as "just decide to be better at chess." The thing is that, to turn "decide to be better at chess" into "actually become a better chess player," you have to play a bunch of games and study openings and probably other stuff. (I can't actually play chess very well.)

Over the past couple of years, I have massively shifted my personality towards four of the five traits you discuss (extroversion, openness, conscientiousness, and positivity). This isn't because I intellectually understood that it would be nice to change. It's because I deliberately practice this stuff all the time.

people who do not naturally possess these traits tend to experience a pretty hefty willpower toll forcing their behavior

True at the start. Becomes less true as you actually gain the traits, and eventually becomes negligible. ("Just forcing" the traits doesn't seem likely to work any better than just playing a squillion chess games. You'll get better results if you focus on specific subskills, ask experts for help, etc etc.)