Interpreting the hard problem as an epistemological problem looks trivial. There's nothing extraordinary in ignorance. The way you describe it implies we have the hard problem of ultrasound too.

I think it's ontological problem: structural description ontologically misses unobservable ideal substance, so the challenge is to provide a principle that would make ideal substance unnecessary. If you don't think too much, ideal substance ontologically solves consciousness, and it's not obvious how mere structural description can live without it.