Can you define "natural necessity"? For Kant, this term has a specific meaning and you don't seem to be using it how he would. For Kant, the need to produce children is a natural necessity. Not sure how wanting to die could be a natural necessity? Other than that, he mostly opposes objective and natural necessity -- arguing that we need to subordinate the our desires and physical needs to the demands of objective morality.
For Kant, one clear case where you can will your own death is when you choose to die doing something heroic. But the trick there is that you are not in fact willing your own death as the core of your maxim -- you are willing something else and accepting the fact that you will your own death. If you consciously will to end your own rationality, this for Kant is per se immoral as it fails the universalization test and the treating humanity as an end test.
Kant more directly addresses your question in the Metaphysics of Morals when considering the duty not to kill yourself and mentioning casuistical questions about whether you can rationally chose to end your life when you know your rationality will be compromised shortly by a disease or other things. Kant leaves unanswered whether you can kill yourself when you know you will go crazy with rabies in the next day or so, but he seems sympathetic to that choice.
A key reason why is that Kant has a type of moral absolutism. That is to say, Kant does not grant exemptions because you got a bad lot in life. So even if you will suffer in agony for weeks on end, it is not at all clear that Kant would grant that you can pre-empt that by ending your life. He specifically distinguishes this from the cases of animals in pain on the grounds that we have a duty to respect our own rationality (this is also the reason Kant thinks lying is wrong in the Metaphysics of Morals)
Contemporary Kantians have tended to disagree with Kant. Christine Korsgaard and others generally try to make a similar move to the one you are suggesting. They try to include "quality of life" considerations but those are generally absent from Kant's own view.
