However, the question of moral ontology remains...do objective moral values exist? Is there anything I (or anyone) should do, independent from what I desire?

Thanks for bringing up that point! You mentioned below your appreciation for desirism, which says inter alia that there are no intrinsic values independent of what agents desire. Nevertheless, I think there is another way of looking at it under desirism that is almost like saying that there are intrinsic values.

Pose the question this way: If I could choose my desires in whole or in part, what set of desires would I be most satisfied with? In general, an agent will be more satisfied with a larger number of satisfiable desires and a smaller number of unsatisfiable desires. Then the usual criteria of desirism apply as a filter.

To the very limited extent that I can modify my desires, I take that near-tautology to mean that, independently from what I currently desire, I should change my mind and enjoy and desire things I never used to, like professional sports, crime novels, and fashion, for popular examples. It would also mean that I should enjoy and desire a broad variety of music and food, and generally be highly curious. And it would mean I should reduce my desires for social status, perfect health as I age, and resolution of difficult philosophical problems.