In any of this there is a slippery slope to be considered, and in many cases it can only be considered on a case by case basis.
The Church teaches that life is sacred from conception to “natural death”.
So we are born, we live, we get a fatal disease and we die naturally. Case closed - But wait -
The Church says that we should not withhold medical treatment if a cure is available. This delays “natural death”.
The Church says we should not withhold feeding tubes. This delays “natural death”.
So, while the Church holds that life is sacred until “Natural Death” and death should not be ended prematurely (assisted suicide), The Church does support the interference with when “natural death” occurs.
I do not post the above to denounce Church teaching, only to note that there are many areas in this field of Teaching that are not so cut and dry as we would like.
My Father died of Alzheimers dementia. At the end he was at home under the care of my dear Mother and of Hospice. Eventually he could no longer swallow. The question of a feeding tube was naturally raised but was rejected because, 1) It would not prevent his brain from continuing to shut down, and 2) It would only prolong his suffering.
My dear mother Loved and Loves my Father with all her heart. They were married for almost 57 years at the time of his death. The decision was made from Love and no other motivation (such as cost). Every one of her 5 children approved of her decision.
My father was allowed to die naturally. It was terrible to watch for, all we could do to help was place bits of ice on his lips to help hydrate a little. He died after 7 days. I was there - My Mother was there - Christ was there.
My mother said, and I agree, that whatever his sins in life, he had done his purgatory on earth.
Now, in the next year or two I may face the same decision as my Dear Wife has this same horrible disease. I am glad for the teachings of the Church on these matters. I am even more glad for the totally Christian example of my parents, especially my mother, throughout their ordeal.
Each case is unique. We must properly form our conscience on Church teaching, but then we must make the best decsions we can in accord with that conscience.