I would like to argue against a mono thematic explanation and term history.

Transhumanism has many different influences. Those include people who were rebelling against church ideology, state doctrine and patriarchy: IUDs, trans rights, dysphoria fixing, medical self help, neurodivergent people, BDSM community member, tech people, people who used technology to function better, people who alter their bodies (previously against norms).

While the Cyborg Manifesto had quite a bit of impact I would like to go back before that. Having artificial limbs, or bone pieces is partially trans humanist and is something that was argued against for centuries. Transhumanism does have some aspects that make it so that people who are discriminated by society can be who they want to be again, wheelchairs, limbs, medication, etc. are relevant.

Of course there is a co optation of transhumanism by capitalist and reactionary elements, who like it primarily for the eugenic aspects. Often those that feel as if they are geniuses themselves and ought to be deserving of all (while lacking the social relations that make life nice).

Especially after more and more of the fruits of labour was given to IT and digital experts, lets say after 2000s and after, many mostly male people who did interact more with screens than people (as they often didn't use their screens to further human connection) do label themselves as trans humanist. Often without the critical reflection of the term or its history.

https://en.wikipedia.org/wiki/A_Cyborg_Manifesto

Haraway began writing the "Manifesto" in 1983 to address the Socialist Review request for American socialist feminists to ponder over the future of socialist feminism in the context of the early Reagan era and the decline of leftist politics.

The first versions of the essay had a strong socialist and European connection that the Socialist Review East Coast Collective found too controversial to publish.

The Berkeley Socialist Review Collective published the essay in 1985 under the editor Jeff Escoffier.[3] The essay was most widely read as part of Haraway's 1991 book Simians, Cyborgs and Women.[28] In 2006, a variorum edition of the Manifesto was published in The International Handbook of Virtual Learning Environments[29] integrating variations from the various versions and returning references and some of the scholarly apparatus that had become separated from the text.

Quotes:

One last image organisms and organismic, holistic politics depend on metaphors of rebirth and invariably call on the resources of reproductive sex.
I would suggest that cyborgs have more to do with regeneration and are suspicious of the reproductive matrix and of most birthing. For salamanders, regeneration after injury, such as the loss of a limb, involves regrowth of structure and restoration of function with the constant possibility of twinning or other odd topographical productions at the site of former injury. The regrown limb can be monstrous, duplicated, potent.
We have all been injured, profoundly. We require regeneration, not rebirth, and the possibilities for our reconstitution include the utopian dream of the hope for a monstrous world without gender.

or

[Materialist perspective on science and humans being evolutionary animals] Within this framework, teaching modern Christian creationism should be fought as a form of child abuse.

I would like to say a few more things.

We all are already cyborgs in some sense. All of us use the net and all of us use computers, phones, whatever as external memory banks, similar but slightly different to how we used books and notes before. We are using glasses to see better, we are using inlays for our shoes so that our body works better, we use chemicals to alter our mood (caffeine), to scrub our teeth and skin…

The aspects that are to be fought in transhumanisms are the regressive and capitalist ones, not the emancipatory ones.

The Empire Strikes Back: A Posttranssexual Manifesto

From Sandy Stone and the late 80s is one of the texts pretty much every trans gender study course will talk about. It is roughly in a similar space to Haraway's book. Concepts and terms like "passing" were worked out and the practice of demanding people to pass critiqued in that text some 40 years ago. Stone was also target of TERFs some 40 years ago.

Ideas of moving robots or animals, beings, stuff animated by spirits is old, too. Aspects and rejections of that find their part in transhumanism.