There can't be one diagnosis because the ridiculous umbrella that Ruth Hunt formulated covers a broad range of people.
For instance - what do these two have in common?
A 13 year old female who does not feel comfortable in her body is essentially normal. If she fixates on the discomfort and it becomes a dysphoria, then she's trans? Not just finding it all a bit weird that her body's doing stuff and she's getting ogled and she's got sexual feelings that she doesn't know what to do about and porn and who'd want to be a woman nowadays?
and
A male in his 50s who is at the top of his career, mortgage paid off, pension sorted, kids grown, wife to be a service human, he's found his "tribe" online and his fetish for wearing his M+S period pants has escalated, and he starts "dressing" openly in the house, then that doesn't cut it for him so he "dresses" outside, then at work, then he's brave and stunning as he always knew himself to be.
Nothing. They've got nothing in common.
One's probably neurodiverse, or traumatised, or gay or overwhelmed. The other has made wanking a lifestyle.
Some definitions would be quite good for trans care, really.