I can’t do anything but let it run its course. However, I feel I am being use a battering ram for DC’s actually being captured by the ideology.
Having written this perhaps I should grateful that I am perceived as the barrier. It’s delaying a life changing decision.
I can relate to this. Not with my own DC, but with a family member I was close to. They fell down the rabbit hole of gender ideology abruptly, in what was to me a clear case of ROGD, which arose as a result of mental health issues, social isolation, and possible autism. I watched them rewrite their entire childhood (which I remembered far more clearly than them) to pretend their gender identity had always existed. I watched them fall victim to predatory influences I could do nothing to keep them from. I watched them recite all the transgender talking points as if they'd been brainwashed by a cult.
I was the only person in this young person's life who refused to play along with the delusion. I challenged it for years. Pushing back with love and logic, hoping I could reason them back to sanity. I experienced the same push and pull you did. The times when they would waver and doubt, they would seek me out and try to provoke me into an argument. I didn't rise to emotion, but I kept pushing back, every time. And they'd go quiet and pull back for a while. Change the subject and become warm to me again. Other times they'd go cold and lash out.
I was the immovable object, and trans ideology was the unstoppable force. And for a while it was a stalemate. At times, I could almost see them coming back to me. The doubt was so obvious, and I always made it seem like their ROGD gender identity was something they could walk back on if they wanted to. Not an intrinsic part of them. The path was always open with me. But it was closed with everyone else. It was a done deal. I was one person, and everyone else was telling them the opposite of what I was.
Telling them they knew themselves best, and their fabricated memories of childhood transness were completely valid. Telling them their mental health issues couldn't possibly be a factor in them feeling like this. That trans couldn't possibly be a social contagion. That if they didn't transition, they'd be miserable for life and probably kill themselves. That anyone who disagreed with trans ideology was a hateful person who didn't have their best interests at heart. And they shouldn't spend time with or listen to such a person, or they'd be contaminated by "far right" bigotry.
Their behaviour became more antagonistic. They tried to cope with the cognitive dissonance of staying in cult-think, by lashing out at the one person who wouldn't let them switch off their brain. I tolerated this for a long time. I felt like you do. That I had to be a human shield for this person I loved, because no-one else would. I kept thinking I could say the right thing and save them. This sweet innocent kid I had loved since I held them in my arms as a baby. But eventually I couldn't do it anymore. I couldn't be one person holding back the flood. There were in their twenties and it had become constant verbal abuse and emotional manipulation. I couldn't get a text from them without crying after reading it. I couldn't think of them without wanting to cry, by the end. I had to step away.
I can't really offer any advice, except to say that I know how you feel. My experience was a few years ago, when the world was in the grip of unquestioning trans mania. I felt very alone in it. But people are starting to wake up now. If you could find others in your DC's life brave enough to push back on the NB identity, then maybe you could make an impact. I hope you can.