I think I am unusual on this board in that I don't have any trouble with definitions of gender identity.
Possibly because I've not thought it through well enough. Don't be shy in saying where you think I am missing something as if you would
Here's how I think about it.
For those of us who are gender non-conforming we know what it means to be gender non-conforming. No issues there. Other people can often see it in us too, to some extent or another, generally dependent on their own sexism or awareness of sexism.
To me, people who say their gender identity is not "cis" are saying the same as saying they consider themselves to be gender non-conforming. It's just that the next generation likes to think up new words for the same thing.
I do believe it is "valid" to live ones life in a gender non-conforming way. I am fine with "valid" too. Whenever I read "valid" I hear "not immoral".
So they say non-binary people are valid, I hear it is not immoral to be gender non-conforming which is fine by me.
I do NOT believe it then follows that anyone "non-cis", i.e. gender non-conforming, is actually the same as a gender conforming member of the opposite sex. I do not believe it makes them indistinguishable from the opposite sex. I do not believe the categories woman and man should be redefined to mean people who are gender conforming for that category plus people who would normally be in the other category but who are officially deemed to be gender non-conforming enough to get a GRC (leaving aside self id).
I don't believe in the redefinition of the categories because (a) why do that? Just why? It will obviously create confusion and for what purpose? (b) it is subjective not objectively measurable and so it is a downgrade (c) it is not a binary like man/woman making it highly impractical to redefine man/woman using gender identity: obviously there will be more than two gender identities, because there are many ways to be gender non-conforming.