Back to @Fishpreferred
I'm going to assess your given definition of gender identity against how well it allows me to communicate who I am.
In my understanding, gender identity is the psychological representation of gender (id est, sex) with which an individual can most readily identify itself. In other words, the sex/gender that a person identifies as being, regardless of the sex/gender of the body.
I identify as being a woman. Ignoring the fact that I now express estrogen and that my body
isn't fully male, let's say that I have a male body as per my genitalia, skeletal structure, and etc.
My identity is, by definition, my concept of what I am. Thus, when asked for my gender
or my sex, I should consult my identity regarding what gender I am, and answer accordingly.
In doing so, I will reply that my sex (and thus, my gender) is male. I do not have any fundamental understanding that I have a female body (nor, in many ways, that I
should have a female body), and thus I do not have any fundamental understanding that my sex/gender is female. As such, by your definition of gender identity, my gender identity is that I am male.
So how, then, do I communicate that I am a woman? Clearly we need another term which addresses this gap in communication; given that sex = gender, then perhaps the three terms we have are not, in fact sufficient.
There are, however, some ways in which I believe that I should have a female body; this is why I am replacing my hormones. But given that my identity is a concept regarding what I am - and not what I
should be - I would argue that even in those contexts my gender identity prior to starting HRT was still that I was male.
In that context, does this definition of gender identity actually have any value at all, other than to describe a very particular kind of delusion - that one's body currently has a different physical form than is observable - which is not at all common among trans people? It's certainly not relevant to my interests in communicating who I am.
Certainly, if none of the available terms are useful we can always make more, but given that the term "gender identity" is
not typically used in the way that you have defined it, I question why you have defined it as such.
Now we're getting somewhere. Why exactly is it more difficult, aside from what is mentioned in your fourth point?
Terms such as "gender identity," "gender role," and "gender expression" are unwieldy and difficult by virtue of being very technical, nuanced ideas; I believe that their nature is too complex for the average person to comprehend. Further, they are terms which are not currently in most people's vocabularies, and as such there is an implied difficulty in that knowledge gap.
These, as well as your closing statement, seem more like reasons not to use it in place of the other terms. If people so frequently confuse and misinterpret the intended meaning, it seems an odd choice of word for your purposes. If you were to coin a new term, rather than redefine an existing one, you would not run this risk (although, I admit, you may have a hard time having it generally recognized).
The fact that it would be difficult to get a new term generally recognized
is the reason why. Which is easier - to accept people's common misuse of a particular term to have a meaning which I find valuable, or to get people to stop using that term entirely and then also use a different one that I've selected?
Myself included, there are a lot of trans people (I'd guess that it's likely most trans people) who don't want to go to great lengths to alter the world to be a place that can better accommodate us, but rather simply want to be part of the world as it currently is. Even in the latter context, there are still many, many things that need to change, particularly instances where the world does not allow us to be part of it. But it's a far more realistic task.
Further, with regard to the term itself, if you're concerned with confusion which is caused by using "gender" to mean too many different things (i.e. to mean sex in addition to social roles), then I question why the insistence that it
shouldn't refer to sex is a cause of that confusion rather than a mitigation of it; given that gender is already being commonly misused by the average person to mean too many things, then I would think that choosing one of them to standardize would be a clarifying step, even
if it is not the "correct" option.
I would think that their specificity would make them more useful than one term.
The utility depends on the context. As most people are binary-identified, the word gender alone can encompass the remaining three terms. I identify as being a woman, believe that many of the roles typically ascribed to women are suitable for me, and express this in a variety of ways, some overt and some subtle.
Stating that my gender is that I am a woman implies all of those things, and does so quickly and efficiently. If I need a brief way to describe who I am, then I need not go into details about my identity, expression, and preferred roles. Very rarely are those details actually relevant.
I do acknowledge that this is not a perfect descriptor, particularly of non-binary people. I haven't really given a lot of thought to how the term gender relates to them, and will consider it.
...a fundamental understanding of the concepts of masculinity and femininity, how they do and do not relate to sex, and how they are and are not opposites?
To clarify, by this I meant "a fundamental understanding of the concepts of masculinity and femininity, the ways in which they relate to sex, the ways in which they don't relate to sex, the ways in which they are opposites, and the ways in which they are not opposites."
