I'm not sure about your "90%" figure. While many who commit suicide suffer from diagnosed or diagnosable mental illness, many do not.
Many commit suicide owing to horrible external circumstances. To turn it around and say it was caused by depression is unreasonable. And in fact, none of the 4 factors you mentioned are "mental illness."
Also, as I noted in this (unscientific) post, mental illness is not in the "TOP 4" reasons among Japanese. Having lived there for almost 2 decades, I can safely say that I do not recall one instance of hearing about a suicide owing to mental illness.
On the other hand, I did hear of suicides owing to the above external factors – too often!
Take it as an example
overweight, fatigue, pot belly and snoring are 4 main symptoms of obesity that is physical illness likewise the 4 factors i mentioned & 4 you mentioned in your post leads to depression, anxiety that are the symptoms of mental illness that effects their mood and behaviour. Remember it is an unnatural death and the intent to die originated within the person so all those horrible external circumstances discussed above must lead a person to a level where they are ready to end their own precious lives, that suggests that a person must be very stressful and depressed condition.
plus, I don't doubt your 2 decades of experience and knowledge of japan, you must better aware of the circumstances leading to suicides there But the comment i added was "factors that adds to it world wide" (not only in japan)including other nations too, to which @azanmalik, @honeysim, @letzsteem agreed to, aware of the caues of suicide in their respective nations.
By the way, really liked your posts about japan.
Have a great day