Just tell me what is practical benefit of having gender and expression detached from biological sex?

It allows for policies protecting people's right to be who they are with out being discriminated against.

My father was a policeman, so here is the problem:
Your ID, please
Wait, you look completely different?!
No, I'm just fluidly expressing my gender...
Sure, sure, you are not masked, up to something :D

You can't change your appearance on a daily bases, because otherwise, it will be impossible to identify any human.

Except it isn't. Sex =/= gender

In my language, and many other languages we don't even have a word for gender?! It was a completely different thing that nobody noticed for thousands of years

From those 30-50.000 how many of them actually want to be treated as a special category?

How many don't want to be discriminated against and treated like dogshit for being whom they are?

I'm telling you, administration is about the efficiency.
If you need to fill billions of various forms and process such data in order to please 1 in 10.000 or 100.000 it will be refused - for practical reasons.

It's the same reason why cars/everyday objects are made by standards. Those things don't fit to 2.15+ not 150- cm height.
Why? Because costs exceed benefits.

just let people be who they are, so long as that doesn't hurt anyone

This is what I say... They can be who they are, so long as that doesn't hurt anyone

Put this in legislation, and you will see all sorts of abuse