I don’t think being trans is a mental illness. I think it’s more nuanced and complex. Obviously the cause is psychological, however to call it a mental illness is likening it to someone with perhaps diminished capacity, which doesn’t apply to trans people.
This may have changed now, but I was taught in university (psychology - about 8 years ago) that there is a typical female/male brain structure. Trans people’s brains align with their biological sex UNTIL they receive hormones and then they begin to change neurologically to match closer (not completely) to their gender identity (this is mapped through MRI scans). So I don’t really believe in the whole “female structured brain born in the wrong body” trope either. I do think there must be a psychological underpinning obviously, but to call it a mental illness would be to assume they are ‘abnormal’ mentally (to use psychological terms), however with treatment and transition, trans people can go on to live as anyone else, so I can’t see it as a mental illness as such because, for example, there’s nothing actually wrong with a male bodied person wearing feminine clothing which is determined by social construction, etc.. I think it starts to become irrational when you have male bodied individuals insisting that they are experiencing a menstrual cycle as a natal female would - that is where the line starts to get blurred in my opinion, when an individual starts to confuse identity and reality.
I support trans individuals, but despise the politics and groups associated. Just in case anyone is wondering what side of the fence I am on.