I believe absolutely that it is for the individual to determine what constitutes a life of enduring and intolerable and irremediable suffering for them. It's very clear that it can be physical suffering or psychological suffering or existential suffering. It's for the person to decide what that will be and to then very carefully lay that out, so that when they are no longer competent and capable of decision-making for themselves, that written request will be able to speak for them.
It need not be at the end stage. They may consider a life of intolerable suffering to come much soon than end-stage dementia, for instance. They're allowed to make that decision.