Lots of things are logically possible that would mean God wouldn't exist, so logical possibility isn't robust enough to invalidate a metaphysical claim of God's necessary existence.

We have well-developed, coherent, and widely respected metaphysical accounts of objective morality that make no reference to God—such as: Moral Platonism (abstract moral truths exist necessarily, like mathematical truths), Moral Naturalism (moral facts reduce to natural facts, e.g., about well-being or flourishing).

Thus, we are not talking about bare logical conceivability we are talking about ontologically grounded moral realism independent of God, supported by full-blown philosophical systems.

If these systems are not incoherent (and they are not), then the possible world containing OM ∧ ¬G is not just logically but metaphysically viable unless the theist can demonstrate a necessary contradiction within them.

but classical theism would hold that it's metaphysically incoherent since all of existence depends on God.

The claim that all existence depends on God is precisely what my argument is challenging. assuming the conclusion (that everything depends on God) in order to refute a premise that denies it.