The truth is that we can never know because the concept of "nature vs nurture" is inherently flawed, it has always been both. So yeah the argument "born this way" is harmful because it is based on a faulty premise. It also is not a positive argument for rights, it is a pleading to not be hurt, which I for one think I bullshit. If I fucking choose to be trans my right to be a trans woman is just as fucking good as anyone else's rights to be however they want and they had better fucking well respect me for it. A lot of this is also cultural, like humans aren't just behaviors that randomly bump into each other, yes be have intrinsic behaviors but they are also deeply wrapped in meaning and language that is not of our own making. So I would argue that a person could have the inclination to be trans but without the cultural collective understanding of that identity they would have no basis for understanding that form of self. In this way the internet has actually been very liberatory in that it has allowed otherwise isolated and rare individuals to come together and communal build culture and identity to reify and externalize their internal inclinations and desires. Like if I had been born as an "upper middle class white man" in the 30's the material and cultural conditions would probably have led me to being a "upper middle class white man" because not only would there been no communally built understood way to articulate my internalized desires, there was the material ease to "be normal" that would have been some degree of salve to the dissociation and distress of being trans. I give this example because I was always fairly certain my grandfather, who lived this "upper middle class white man" life, was some form of queer, but never articulated it as such. Having read my grandfathers journals after he passed I can get the sense that this was true, though he never comes out and states it plainly as such.

Which actually brings up the concept of dysphoria, this as a condition only exists in our communal understanding of it as such. What is the concept of happy? Mentally well? Mentally unwell? Many of these concepts are not some externalized force of natural but a communally decided upon condition. You can correlate certain brain states to certain phenomenology, but that doesn't constitute an unhealthy or unhappy mental condition. A woman who is working 3 jobs and living in a shithold apartment to take care of her kid probably has a brain state indicative of depression and distress, but no sane person would say she has a "mental health problem" her mental health is 100% appropriate to the shitty conditions she is forced to live under. It is in fact unhealthy to be happy in states of distress, the man laughing and screaming that he is having a wonderful time while getting shelled in a fox hole is not a healthy man.

As to your connection to your past self? I guess that is a project we need to work on together. I feel the same conflict that it seems most trans people do toward their past selves. On the one hand I like to say "I killed [deadname]" at the same time I am proud of many of the things that person achieved, even if I didn't particularly like being them at the time. Reconciling that is hard for me largely because there is anger at other people and society about how I was failed. All those therapists that no once asked me about my gender or identity when I was a small scared child, all the schools that let me be abused and bullied for what I perceived a just being myself.

Sorry for the long post but it just kind of poured out.