The core principle of the pro-transgender argument is Freedom of Expression (supported widely).

Tossing aside bias entirely (I am pro-trans), I see the angle you're coming from; however I disagree. The terminology used for a transgendered person is tricky, the general consensus is that you should accept their outwards expression; if a person presents as a female, the term is she/her, if they present as male, it's he/him. When referring to life before sexual reassignment surgery (SRS), people generally refer to them as their biological gender.

since now anybody who says they are what they think they are can walk into a bathroom of their choice.

The bathroom issue is a biggie for transgendered people. It varies based on location, but what most people do is to use a unisex bathroom when one is available. It removes the stigma of, for example, a pre-op transwoman using a female bathroom - biological females can feel awkward knowing a biological male is in the bathroom, and the transwoman can feel awkward being in a restroom with males. Hence, unisex when available.

I just feel like this conversation wouldn't even be a thing if we didn't have the science to change sexes.

I respect this idea, however, the history of crossdressing/transgenderism is long; Charles-Geneviève-Louis-Auguste-André-Timothée Éon de Beaumont was alive in the late 1700's, well before the science to change sexes existed, and he lived as a woman for the later half of his life. The discussion we're having would have happened then as well, because it was less socially acceptable for a man to dress as a woman than it is now.