testosterone is the way to do it. If they set a cap of 1000 "widgets" of testosterone, then athletes who would exceed this would do all sorts of unhealthy things to their bodies so that is they can weigh in at 999 widgets and be allowed to compete. It still doesn't level the playing field. What about athletes that naturally have 999 widgets of testosterone? Suddenly they have the unfair advantage.

The delineation needs to be obvious and immoveable. Birth gender or chromosomal gender are clear. You are either XX, XY, or some weird aberration. You have to compete in one of those three categories. If someone is XX but has a natural condition that gives them high testosterone, then I think they should still be allowed to compete with XX.

Elite sports are by definition a freak show. They celebrate amazingly awesome physical and mental gifts. I don't like the idea of a committee deciding which freakish aberrations are to be celebrated and which are to be ostracized.