If we exclude sci-fantasy bullshit like uploading your mind onto a mech or submerging yourself into a pool of nanobots to rebuild your entire body on a molecular level, I mostly take transhumanism to mean having a general optimistic view of technology and believing technology will reach a point where people will proactively replace most of their natural body with artificial equivalents. In general, we either use technology as a means of augmenting our natural bodies or reactively replacing parts of our natural bodies that's deficient. A cane is a basic form of augmenting our arms, in this case how far our arms can reach. A prosthetic limb is a replacement for a deficiency, in this case the lack of an arm. A wheelchair augments our bodies by enabling us to quickly travel on flat surfaces with our arms. The fact that it's mostly used by disabled people who have difficulty moving with their legs doesn't change the fact that it's mostly a form of augmentation rather than replacement. The arms are augmented in order to compensate for the legs' deficiency. An actual replacement would be prosthetic legs because prosthetic legs don't make sense if you still have natural legs.

But I don't consider this transhumanism. Transhumanism would be when people start surgically removing their natural limbs in order to get better cyborg limbs or replacing their natural eyes with cybernetic eyes that can see infrared and ultraviolet. The reason why I wouldn't consider myself a transhumanist is I'm far more pessimistic in the tech actually being able to reach a point where people would knowingly consent to do this. So far, actually existing transhumanism is essentially just cosmetic surgery. But that's the thing. It's all mostly skin-deep. Once you're talking about replacing natural hearts with better synthetic ones or replacing your legs with metal spider legs, you start running into issues. Take phantom limb pain. Phantom limb pain occurs when someone lost a limb but still has pain as if the limb were still there. With this in mind, how many abled people would actually sign up to replace their natural arms with cybernetic ones? I could see amputees signing up since they already don't have the limb and already feel phantom limb pain anyways. If anything, the fact that their cybernetic limbs outperform their natural limbs is just compensation for the fact that they traumatically lost their limbs and have to feel phantom limb pain.

Ultimately, I personally don't think we'll reach a point where those replacements won't come with various side effects, meaning the only people who would actually take those replacements are people who need the replacements anyways.