If you want to just use the word 'belief' to also describe moral principles that seems okay as long as you don't confuse them with beliefs proper.

The reason I want to do this is because things like logically manipulating moral beliefs / preferences in conjunction with factual beliefs / anticipations makes sense.

But I think this is our disagreement:

A utility function doesn't constrain future experiences. Thats the reason for the conceptual distinction between beliefs and preferences. The projection of our utility function onto our map of the external world (which turns the utility function into a set of beliefs) is illegitimate because it isn't a reliable way of forming accurate beliefs that correspond to the territory.

You say it's illegitimate because it doesn't constrain future experiences. If it constrained future experiences incorrectly, I would agree that it was illegitimate. If it was trying to constrain future experiences and failing, that would also be illegitimate.

But the point of morality is not to constrain our experiences. The point of morality is to constrain our actions. And it does that quite well.