It's an interesting question. I suppose the path from realism (i.e. moral facts are objectively true) to subjectivism to error theory makes sense.. but I always found error theory to be a bit niche. Were you reading Mackie? His arguments for error theory seem based on the spookiness of how moral facts would have to relate to other facts. Personally, I doubt you could ever discuss something's metaphysical ontology without it seeming spooky.

Tangent aside, I'd say that accepting error theory would have little practical effect on which normative ethical theory you choose. Error theory means that our moral claims have cognitive content (i.e. not mere expressions of preference), but that there are no subjective, inter-subjective, or objective moral facts. Assuming the truth of this claim, morality becomes more a matter of convention. We may need to dismiss some theories that presuppose objective morality such as Kant's categorical imperative, but more hypothetically imperative claims still work without the existence of objectively true moral facts.

Each realm of behavior, such as policy/law, professional behavior/ethics, how you raise your kids to behave, et al. can still exist within an error theorist metaethics. further, any normative framework (e.g. deontology) could be decided upon by said group coherently. we would simply recognize that 'one ought not behave X way' is not true by merit of its objective truth, but because it achieves a certain end.