I'm with you on utilitarianism. But it still takes some assumptions, even ignoring aggregation.
1. Human life matters
2. The quality of human life matters
Sentient life, but yes. It requires that assumption. If you insist upon that assumption, though, then you run into all weird things. "Healthiness" doesn't exist, for example.
It's a bit circular, but sentient life always cares about the quality of sentient life. The quality of life matters as long as there's sentient life. Who says? Well, every sentient thing that exists. It's universally true. As soon as you have sentience, you the concept of 'good'. As soon as you have actors capable of moral actions, you have morality.
Take subjective thing:
Say its a result of the makeup of the brain, which is physical.
Now it's objective.
Objective/Subjective is somewhat meaningless, I think, if you believe that all thought originates from physical processes (as I do).
What's happening is that the subjective thing operates according to objective laws. But no, there are still subjective and objective things.
Take, for example, the experience 'tasty'. It's completely subjective thing. There is no possible viewpoint from which to say I'm wrong or right about whether I think something is tasty. Sure, whether I find something tasty operates according to objective laws, but it remains subjective. The objective truth is determined by the subjective truth.
'Harm' is separate. Whether something harmed someone is independent of their perspective, except insofar than their perspective is a factor (but, here, their perspective operates according to objective laws (e.g, giving a flower to someone who likes flowers is better than giving her a spider, objectively)). In fact, the more objective a viewpoint you get, the easier it is to determine if harm was caused. The
in toto answer will be shaken out by the universe.
The morality then kicks in because the action was due to a decision made. This means that the action (and its result) can be compared to all other possible actions, and then sorted. And, since morality is civilizational, one person's decisions are affected by everyone else's decisions (as well as their constraints). Moralities can then be judged by how they sum as they're applied universally, as the number of actors increase.
I always go back to 'the circle'. Is the circle mathematical concept subjective or objective? Is pi subjective because some people thought it was 22/7? Is it impossible to compare the 'circleness' of two objects?
If people want to say circles are a human creation, then sure. I can see that, and I can agree that so is morality. But, if I then asked "what is morality?" and you said "it's the rules we follow in order to determine whether what we're making is circular or not"
Circles were created by people, but they came out of evolved heuristics that well-predate the first calculation of pi. Obviously morality is vastly more multi-dimensional.