Occupational therapist here. About 80% of people use a tripod (which is considered the “right” way to hold a pencil) or quadropod grasp. A tripod fingertip grasp is technically the “best” because it makes the most efficient use of the musculature of the hand. When you hold a pencil with the tips of your first two fingers opposite the tip of your thumb, this uses the smallest and fewest muscles to control the movement of the pencil. The closer the pencil moves to the base of your fingers, the more and larger muscles you will need to use to move it around, which leads to fatigue and, sometimes, pain with extended writing.
There’s the technical side. Now for the functional reality.
First, 20% of people DON’T use a tripod grasp, and this is not a sign of neurological differences. It’s true there is a correlation between autism and difficulty with motor planning, but one does not equal the other.
Second, since our society does not actually require extensive handwritten text anymore, this skill is just not as vital as it once was. Lots of (especially older) teachers and OT colleagues may disagree with me, but occupational therapists are supposed to focus on a person’s ability to functionally participate in work, play, and self-care, not attempt to make them do things “the right way.”
Third, if your pencil grasp IS causing you pain or excess fatigue because of the demands of your job, school, or hobbies, occupational therapy can help. We do this by teaching a different pencil grasp (which OP has tried) or modifying the task (different types of writing tools/pencil grips, positioning the paper on a slanted surface, typing, or even voice-to-text).