Human morality is immensely complicated, because human brains are a complicated, incoherent mishmash of innumerable interacting preferences and experiences.

The most promising defence of realism is that moral value (disvalue) and positive (negative) experience are simply one and the same thing.

How do we know? We have direct access to the fundamental nature of subjective experiences and perceive that positive (negative) feelings have the property of goodness (badness). This can at a stroke solve the is-ought problem, the 'open question' puzzle, and the mystery of how we could have knowledge of causally inefficacious moral facts.

This book 'The Feeling of Value' lays out a plausible moral realism grounded in phenomenal experience:

Sounds descriptively false? I prefer lots of things that aren't a matter of valenced experience.

You don't need to have "direct experience" of all moral properties in order to be a moral realist, any more than you need direct experience of porcupines in order to be a porcupine realist. You can just acknowledge that moral knowledge is inferred indirectly, the same as our knowledge of most things. Seems like a classic case of philosophers reaching weird conclusions because they're desperate for certainty (rather than embracing Bayesian/probabilistic ways of thinking about stuff).

Likewise, you don't need direct experience or certainty in order to reconcile "is" and "ought". Just accept that "ought" facts look like hypothetical imperatives that we happen to care about a lot, or look like rules-of-a-game that we happen to deeply endorse everyone always playing. No deep riddles are created by the fact that "what is a legal move in chess?" is not reducible to conjunctions of claims about our universe's boundary conditions and laws of physics; we just treat chess-claims like math/logic claims and carry on with our lives. Treating our moral claims (insofar as they're coherent and consistent) in the same way raises no special difficulties.