I am so sorry OP. I don't know if I have the answers for you but you may find my story relatable.
My mother was in almost exactly the same situation as you two years ago when my 19 year old brother decided out of no where that he was a girl. A combination of a feeling like a misfit and having poor socialisation growing up meant he was particularly impressionable to the diagnosis his radical, belligerent progressive girlfriend gave him during covid. Lockdowns gave him two years to obsess over his new identity and he boasted how easy it was to get hormones after only three health care appointments.
I, unfortunately, was fully on the trans bandwagon at the time and celebrated his newfound identity. Worried that he would kill himself, I painted a rosy picture of his future as a trans person. Mum and dad didn't know anything! They were closed-minded conservatives!
No surprise, my brother quickly cut mum out of his life once he realised she didn't truly believe he was a woman, and so I kept my mouth shut to avoid the same fate. Mum has been so devastated, always asking me for news about her son, paying for his phone bills and private health care from a distance, terrified to let go. I try to encourage him to see mum's perspective, but he doesn't need her. Gen Z is bloodthirsty when it comes to kids cutting off parents. It's normal for teenagers and 20-somethings to undervalue their parents relationships in their lives, but this takes it to a whole new level.
Dad, never tolerated the trans thing and immediately laid down the law - "no dresses, female pronouns in my house" and my parents clearly have conflict with each other over these differing approaches. Dad thinks mum and I are enabling.
It seems a lose-lose situation; Hurry them down the path with your consent or lose them forever. You face judgement from people on both sides no matter what you do.
I try to straddle the line between these two options. I take a more watchful waiting approach... caution against permanent action and dangerous activity but I have so far been unsuccessful. He's too far along. He truly believes he passes as a women due to the one-sided confirmation of friends, employees etc. He has not considered fertility or children, despite it probably being too late. He is still in a cloud of "euphoria" with his transition. His peers celebrate him and go OUT of their way to make conversation with him because it gives them a veneer of acceptance. He does not have the social awareness to see these things as self-serving and fake, or that this attention won't last forever. Once the next big social cause comes along, will he emerge from his fog and realise he has all but ruined his chances at a normal life? I suspect so. And its in part my fault because I contributed to this by playing along because that's what twitter, YouTube, Reddit told me I had to do to be a good person...
I don't know what you should do about your daughter. Do you feel you know her well? Do you feel like she aspires to be like you or sees you as a role model? Is there a chance that while you watchfully wait, you could really encourage her to talk to you? Coming from the perspective of just really wanting to understand her mind. It might make her more receptive and understand that you only want the best for her. She won't be able to say 'Mum just doesn't get me, or know how I'm feeling" if you really seek to understand her perspective (not agreeing!).
Your influence is limited as she is an adult (so is my brother), but you are still her mother and undoubtedly an important force in her life or she wouldn't be trying to persuade you. The most important thing is that you make sure that she considers a continued relationship with you to be WORTH it to her - no preaching, no pressuring, no guilt-tripping. And I also think it will help to really bridge the generational gap between you right now. If she's anything like my brother, she has discounted all your opinions as antiquated, bigoted, uneducated and (most ironically) naïve about the true nature of the world. Its important that she sees you as being on her level. Something you could do is go through young photos of yourself as a teen or 20-something year old. Emphasise the ways that you were similar to her at that age. Sounds stupid, but I found that it really made me take my parents perspective more seriously when I saw proof they used to be young and cool like me haha! They'd been there, done that - and their advice had to count for something.
This is such a difficult position OP, I hope you can pull through. Your daughter is being let down by the mental health industry, government, school systems... forces much stronger than you or your husband. But your child is still a child, her brain not even fully formed, with lots of growing to do. Young people hold beliefs fast and loose and she she may be more willing to change her perspective that she'd be willing to admit.