I think there's a lot in stoicism - accepting what you cannot control.
My children are younger. One day I was in another room and heard a noise. Then older kid says "you've broken the TV". They didn't sound hurt so I didn't immediately rush through. My youngest comes through utterly distraught and tells me what happened.
What I was amazed by was having even a short gap (under 30 secs) where I accepted the TV was broken (thankfully insured!) before he came through meant I was able to respond calmly and kindly to a child who had accidentally damaged something and was honest and brave in coming and taking responsibility for it. My first response could be "are you hurt?".
That gap allowed me to respond in a way I was proud of. If I had been in the room, trying to get him to stop being wild or admonish him to be careful and been too late, then my impotent frustration would made it difficult to be that clear and calm. I'd have told him off, perhaps angrily and his attention would have been on my reaction not on what he had done.
Back to your daughter. If she had already taken testosterone - or even something non trans related like gotten a facial tattoo - and then regretted it, then I think you could, like my young kid with the broken TV, find that you could respond with compassion to a wrong choice already made. Because there is acceptance that it can't be undone so your focus moves instead to maintaining a good relationship and problem solving with them how we move on from here.
But because instead of having already occured, you are seeing her teetering on the edge of a decision you think she'll one day regret, you are full of frustration because if she'd only do what you tell her too then she could save herself that pain. So you (and your husband) are distressed and trying to do all you can to get her to see it's a wrong choice to prevent disaster. You have the illusion that it is something you can stop if you just say the right thing etc.
Accepting that it is a choice she could make whatever you say or do. Forgiving yourself for not knowing what the magic words are(if any) to stop it. Focusing on adapting to the parent of an adult relationship and it's sorrow that we can't protect them from everything forever, even themselves. Staying a reliable place for her to come back to and acting within your own integrity which might mean not cheering her on self harming her body, whatever society says. I think that's all you can do.
A quote from Harry Potter that comes to mind, related to the estranged young adult Percy Weasley who hasn't resumed contact with his parents even after its clear they were right is: "Dumbledore says people find it far easier to forgive others for being wrong than being right."
Good luck with it all.