'If Dementia counted as a valid reason to legally top yourself (or your relatives) then so would depression, PTSD, BPD etc.'
No, because all of these disorders are treatable. Dementia isn't. (It is true that sometimes a treatable disorder such as depression in an older person is misdiagnosed as dementia; and obviously it is important that the diagnosis is confirmed as definite before assisted suicide is even considered.
'Where do we draw the line?'
Surely we draw it where a disorder is (a) incurable and progressive; (b) has reached the point of causing unbearable suffering and/or loss of dignity to the person; and (c) where the person has currently, or in the case of loss of capacity previously, recorded a wish not to be kept alive under such conditions.
'There are also Cancers where even a terminal prognosis could conceivably lead to many years of healthy life (bowel Cancer / brain cancer / some types of blood cancers), would those qualify? '
Nobody to my knowledge has ever suggested that people with forms of cancer that can be kept at bay for long periods (even if unlikely to be permanently cured) should undergo assisted suicide/ euthanasia. It is people in the last, irreversible stages of cancer or other terminal diseases who may wish for this solution.
One general point: Suicide as such is legal. What is not currently legal is to assist another person in killing themselves. Thus, someone who wishes to die, whether because of a medical condition or any other reason, can usually choose a way to do so - sadly in the case where this is due to a temporary and treatable disorder such as depression. The issue of assisted suicide comes up with regard to people who are already so ill or incapacitated that they cannot take independent steps to kill themselves. Forbidding assisted suicide may actually cause some people to die much earlier, by killing themselves or (if they can even afford it) travelling to Switzerland while they are still capable of doing so, rather than wait for a time when they are too incapacitated to do so.
This is a sad topic, but we cannot pretend that it doesn't exist.