I would argue someone is not mentally ill when they have received treatment and are functioning normally again. I have OCD and there was a time where I lost hours everyday to compulsions. I had therapy and I’m better now. Would you say I’m mentally ill?
If you don't need therapy anymore to function normally, then no, you're not ill anymore. By contrast, I still need my meds to keep my depression under control and function normally, so I'm still mentally ill.
Gender dysphoria is the mental illness and that is what causes the distress.
Yes, and treating it with surgeries and hormones is the current accepted treatment but very likely not the best one, far from it. We should be looking for something FAR less invasive, but TRAs won't let such research happen.
Many posters here would say they had gender dysphoria but aren’t trans.
Heh? I'm not sure I would agree with that. Some posters have shared that they went through a gender dysphoric episode as teens, but they typically no longer feel it anymore.
It’s possible for gender dysphoria to reduce or go away with transition, so I don’t know if I’d say ‘being trans’ on its own is a mental illness.
Most, if not all, trans people still feel some degree of gender dysphoria after transition, simply because they are not the opposite sex and never will be. Whether it's dysphoria induced by the still "deficient" body, or the memories in the "wrong" sex, or the impossible fertility in the "right" sex, or the sexuality/romance problems induced by being trans, or the occasional clocking, or whatever else, there remains plenty of sources of dysphoria, and it's a very lucky trans person indeed who never experiences dysphoria again after transition. And since you agree that gender dysphoria is a mental illness, then trans people are mentally ill by definition.