I do not believe that death necessarily has to be painful at all. You've stated that natural instincts manifest in different degrees for different individuals. A normalfag would probably experience a huge amount of agony if they were not able to engage in social interaction regularly whereas this would actually be a luxury for many wizards. These compulsions and aversions originate, I believe, from value/desire systems. We seek out food because we desire satiation therefore place value on it. Conversely, the opposite of this desire, hunger, is given a negative value. This duality can be seen in all desires: comfort and pain, security and anxiety, prestige and disgrace. We usually think of these desires as being fundamental pieces of our personality, but it is evident that they are mutable. Our goals and desires change all the time. There is evidence in those who do go through with suicide for philosophical reasons that even the fundamental desire, the desire for continued life, can potentially be altered through rational analysis and self-discourse.
But what is suffering? Suffering is when you fail to achieve your desires or avoid your aversions. Because desires and aversions are mutable, that which causes suffering is also mutable. Of course, physiological pain also exists, but there are methods of suicide that are completely free of physical pain. And it is perfectly possible to endure pain without necessarily suffering a great deal from it, especially if you know that you're in control and it will be over quickly. The suffering that is brought up most often is the suffering of contradicting the desire to continue living. If the antinatalist worldview that life is a moral negative is followed through completely, there is no rational reason to maintain any desire for continued life. To maintain such a desire would be contradictory with the beliefs and values consciously accepted.