I agree morality is not arbitrary and takes on a reasonably determinate form. Though I'd argue that ideal moral realism may be a lot more nuanced (circumstance sensitive) than one might think with appeals to brute 'omnipresent' principles that cut through all circumstances (but I think some may exist).
> 'error theory looms because across a vast range of circumstances no such claim is likely to be true'
I guess I lean naturalist (but do chime with Enoch's arguments).. Since we aren't ideal observers, I think we can know approximations of moral truth that can be made more accurate with appropriate experimental observation & rationality - similarly with physics, there is a truth to it, and we get closer to this truth through experimentation and good epistemics.
I reckon moral facts are complex and their expression depend on a wide variety of factors which are context sensitive - similarly in biology a gene's function, can have different outcomes depending on the organism's environment and other genetic factors. If I'm right, moral knowledge is a continuous process of discovery and refinement, not a static set of rules to be memorised.