This is your belief.
I started my transition over 40 years ago in school. I had to leave school because it was impossible to exist without constant abuse and violence. I persisted even though my changing was against considerable adult efforts to make me 'normal' and sometimes begrudgingly gay when 'normal' was a bridge too far. I have spent over 40 years in womens spaces. In toilets, changing rooms, shared sleeping accommodations. The gender dysphoria has been gone for decades and I have never regretted my decision. I assume everyone feels this at peace with their lives.
Based on my own experience, I used to believe that children and adolescents absolutely know if they are trans. And if we go to the effort of upending our lives at this age, and transition, it is almost a guarantee that we will be living the rest of our lives as that gender.
I agree that there can be no philosophising to dismiss this very real issue. The gender critical mischaracterisation of a minor MTF's transition as some goal-focused perverted action to 'gain access to girls' is cynical as hell and does no one any good. Could it happen? I guess. There are some awful people in the world. I believe I would be able to spot a bad faith attempt at transition. I believe detransitioners genuinely felt gender dysphoria at a time but eventually worked through the cause and decided transition was wrong for them. They are still a minorty of people who transition.
There used to be just as many trans boys as trans girls in the news for requesting accommodation in their schools. I assume that number has shifted to a majority of trans boys asking for accommodation, based on all the campaigns and websites against 'ROGD.'
I do believe trans girls are girls and trans boys are boys. I am not gender critical. I support case by case inclusion because I was not allowed an education. The fact that it was even allowed and culturally supported for any length of time warms my heart.