I think this argument, in order to work, needs some further premise to the effect that a decision only counts as "definitive" if it is universal,

Ok, but it would have been helpful to have argued the point.

if in some suitable sense everyone would/should arrive at the same decision; and then the second step ("Morality tells you what you should do") needs to say explicitly that morality does this universally.

AFAICT, it is only necessary for to have the same decision across a certain reference class, not universally.

In that case, the argument works -- but, I think, it works in a rather uninteresting way because the real work is being done by defining "morality" to be universal. It comes down to this: If we define "morality" to be universal, then no account of morality that doesn't make it universal will do. Which is true enough, but doesn't really tell us anything we didn't already know.

Who is defining morality to be universal? I dont think it is me. I think my argument works in a fairly general sense. If morality is a ragbag of values, then in the general case it is going to contain contradictions, and that will stop you making any kind of decision based on it.