You can’t rely on someone who is suffering to make a rational decision about weighing a very permanent choice with the chances of maybe someday getting better.

That’s just not true. People dealing with chronic pain can absolutely make informed decisions about their own healthcare, including voluntary euthanasia. Psychiatric and neurological illnesses could potentially impair a person’s judgment enough to bar them from making the choice themselves, but this notion that anyone who is “suffering” can’t be relied upon to make a rational decision because they’re somehow too biased by their own pain is pure idiocy.