My heart goes out to anyone also in this situation.
While I am not sure exactly how relevant my experience might be to your situation, for me, this stuff is the hardest parenting has been. It is emotionally exhausting and a marathon not a sprint, so I would advise anyone starting out to try to pace themselves.
'Put on your own oxygen mask first' before helping a child. Self care is sooo important because it takes very little until everything is revolving around the ROGD kid.
Instead, make the effort to actively build in things that bring you joy and that give you life.
Try to have a consistent approach with your partner.
Focus on any other kids, to make sure they are coping OK and are able to talk about how they feel. Their history is also being rewritten and they need to be able to call BS, where necessary. "No, you didn't 'always love' High School Musical. You chose to head off upstairs to play with your Bionicles when it was on and that was also fine."
It's not easy but, while being sympathetic to the dysphoria, aim to be the Upholder of Truth and the Bringer back to Reality.
Sound out a couple of people in real life to whom you can chat freely about the challenges.
If you find yourself spending a lot of time researching this topic, keep reminding yourself to make a distinction between the activists and your kid. Your kid is not Karen White. They are not Mermaids or Stonewall. Your kid's vulnerabilities have made this ideology seem appealing. They are still vulnerable.
Online ROGD support groups can help.
Consider 'muting' anyone who is over-excited about your son turning into your 'new daughter'. This is performative and about them, not you.
Avoid talking about gender to the ROGD kid as far as is humanly possible because any criticism of the general ideology is taken as a direct attack on their personal identity. This is very difficult when it has become their special interest.
Conversations with a male 'identifying as a woman' NEVER go well when you are on your period!
Instead, go all out to encourage critical thinking about things in general and call out any misogyny, homophobia, racism etc where you see it. Occasionally the cognitive dissonance will jar for them too.
It's not your fault.
A few years ago these kids might have been an emo or a goth or a hippy or a punk in order to express how 'not like the other kids' they were. They might have told you that they were gay. You would have been absolutely fine with that.
Now 'non conformity' (it is paradoxically super conformist to gender stereotypes) comes with the threat of a frightening dollop of hormones and serious surgery, with a side of authoritarian dogma. And yet somehow we are the creepy ones for being concerned about the loss of fertility, sexual pleasure and increased health risks...
We are officially in the upside-down.
If you had seen many (any?!) signs of gender non conformity in the years leading up to their trans declaration, then it would not look like ROGD.
This is what the more vocal trans activists don't understand: this is a NEW thing in these kids' lives.
Mine initially said he had started feeling dysphoric for "just a couple of months" during a period when we already knew he was going through social isolation, bullying and exam stress and spending a huge amount of time in his room online, (has ADD, highly likely ASD). Later on he realised that this was not the orthodox position and changed the narrative to one where he had 'always felt this way'.
As parents, we are more than aware of the bullying, the ASD, the AD(H)D, the other mental health issues and life stresses that have already made life difficult for these kids. They have enough on their plate already.
Transition therefore seems like a magic wand. All your problems will go away! Rewrite your past! Acceptance! Nothing you can do or say can be questioned because 'transphobia'!
The anguish for us comes because of the wholesale rejection of the old identity, which as typical loving parents we were part of co-creating, (modelling values, building happy memories etc.) In contrast, the brand new identity which denies the past and material reality, seems utterly inauthentic.
They may indeed no longer be a very nice person but they are a hurting person and somewhere in there still is the person you love.