You're not following your own principle.

If trans women are women is the guiding principle, then the conversation about testosterone is not a valid conversation, because testosterone levels don't control whether one is a trans woman or not (I guess unless you're very very strongly in a strictly transmed camp--but in that case a lot of so-called "trans women" are not trans women).

Perhaps you mean that it is one of several central principles, and if so, perhaps clarifying what the other principles are will make it clearer what approach makes the most sense.