It seems to me that this ascribes an existence to “I” that is separate from the brain; with no evidence for this existence, that makes it mystical/magical thinking, a.k.a. superstition.
Not really. The "vertiginous question" is just that, a question. We can't call a question superstition because we don't have a good answer for it yet.
For example, we can't call the question "why does gravity exist" superstition either. It's a valid question. We can feel the gravity, measure it, and forecast it, therefore it exists, but we still don't have a concrete answer as to what causes it. We don't assume that there is a metaphysical explanation, but we don't know the actual answer either. Similarly, the vertiginous question is a meaningful question, even though we don't have an answer.
For example, we can't call the question "why does gravity exist" superstition either. It's a valid question. We can feel the gravity, measure it, and forecast it, therefore it exists, but we still don't have a concrete answer as to what causes it. We don't assume that there is a metaphysical explanation, but we don't know the actual answer either. Similarly, the vertiginous question is a meaningful question, even though we don't have an answer.