From your source:
The rates of depressive symptoms (51.4% for transgender women; 48.3% for transgender men) and anxiety (40.4% for transgender women; 47.5% for transgender men) within the current study far surpass the rates of those for the general population.
Again, you are posting sources which don’t really agree with you. 51% is certainly not a “huge majority” much less “astronomical.” Further, having depressive symptoms is not actually the same as having a mental illness. It just means that you demonstrate some symptoms which coincide with mental illness. That’s an important distinction.
Your source absolutely does not support the notion that, in your exact words, “a huge majority of transgender people suffer from mental illness.”
More accurately, it would be stated that transgender people suffer from certain symptoms at a rate which is higher than the general population, but from a purely logical perspective, this would be expected. For most of history, transgender people have been ostracized and told that their views of themselves are “wrong”. That would, naturally, create a major source of stress and depression for them.
Further, that specific study does not at all suggest that their transgenderism itself is a mental illness. It’s talking about how to treat them for those symptoms while they undergo their transition. That implies that the doctors writing it do not view their transgenderism as an illness at all.
You would need to provide such evidence.