This reminds me of an interview with neurosurgeon and author Henry Marsh who had prostate cancer.
He described how he's arranged to end his own life should he get alzheimer's or dementia as he didn't want to waste away. But he explained that he has access to knowledge and things ordinary people don't.
I looked into this recently and it seems like it is basically impossible to pre-arrange assisted suicide for alzheimer's or dementia. Even in countries which allow death with dignity. I find it very strange because it’s so common and I’m sure many people would prefer DWD in those circumstances.