I also agree with the vast majority of what you said, and really like a lot of your reasoning. I think the one real issue I have is that I think you're prioritizing what I think ultimately amounts to an "invalid" perspective.

My basic thought process is this:

1. An ought implies a goal

2. A goal implies a sentient perspective with a "will" or "desire" towards that goal.

3. The only sentient, willful perspective we know of is our own.

4. Therefore, the perspective we must use to establish our goals and our "oughts" has to be our own perspective, and not some purely rhetorical perspective of "the body", or of "nature".

Essentially, I think your argument violates the fourth point above.

That's my thesis in a nutshell. But I'll respond to some specific quotes of yours to clarify what I mean.

"It would be an instance of the naturalistic fallacy to say that because the child wanted to hit Billy, they should have hit Billy."

I'm pretty sure my proposal for defining an "ought" and a goal avoids any fallacies in this regard.

The child has a willful perspective, from which we can presume a goal of feeling good. Human lives are complicated, so it's tough to say what the actual effect of this child's hitting Billy would have over the short and long term. All they can do is make their best guess at the time, and if their calculated expected value of hitting Billy ends up looking positive for their overall feeling good, then yes, they really ought to hit Billy.

Of course, Billy has his own sentient perspective, from which we can establish a separate goal and set of "oughts". If Billy's own calculations tell him he won't enjoy being hit, he "ought" to try and prevent it. And if there is an adult in the room, they'll have to make their own calculations as well (e.g. "will Billy's parents be mad at me if I don't intervene?").

There are many sentient perspectives, and therefore as many goals, and bases for "oughts" in the universe. All goals, and therefore, all "oughts", are relative to the sentient perspective from which we established the goal.

"As a matter of an imperative it fails because it doesn't explain why we ought to be that way. And as a matter of necessity it fails because we have such incredible potential to work within the (weak) bounds of our biological constraints that you can't get morality out of it."

I'm not saying we "ought" to feel good. What I'm saying is that, as a matter of pure, introspectively observable fact, our goal is to experience good feelings. And our "oughts" simply follow from that. I don't choose to want to feel good; I just want it. I have a will, but it's not a truly "free" will in this respect.

You are right that we have a lot of wiggle room as to how to focus our attention and arrange our lives to go about trying to feel good. Our individual, daily "oughts" are indeed shifting and hard to determine. But the unchosen goal of feeling good is unchanged by this fact.

"However, if nobody is having children, this is not perfectly fine. This would be a crisis of the most profound importance."

From whose perspective? Certainly not from the universe at large, since the universe has no sentient perspective to care. And certainly also not from "humanity's" perspective either. Humanity, as a unified whole, has no common sentient perspective.

So the only perspective from which it has any import is from the perspective of individual humans, who presumably have some sadness / fear button that gets triggered by the prospect of human extinction. Like most humans, I have that button. I have a pretty involuntary, adverse reaction to the idea of human extinction.

But the crisis is only a crisis because it affects my (and other peoples') happiness. There's no other way we can ascribe any meaning to it. The only way avoiding extinction becomes an "ought" is because it conflicts with some of our semi-hardwired ideals for feeling good.

"The point of happiness and sadness is to allow us to behave in a way that perpetuates us."

Certainly, happiness and sadness are only relevant to the body's likelihood of survival insofar as they affect the body's behavior.

But the body is not sentient, and so has no will or goals from which we can derive an "ought".

This is what I mean when I say that I think you're taking an "invalid" perspective. Sure we can talk rhetorically about the body to say that it has a "goal" of survival, and that our sentient feelings are "mere means" from the "perspective" of the body.

But although the body gives rise to a sentient perspective, the body is not itself a sentient perspective. And so it "the body" is not a valid perspective from which we can establish a goal and resulting "oughts". Our goals have to stem from a sentient, willful perspective itself, not merely on our mental model of what sustains it.

"But so too is there an immensely profound danger of not going with (3): anything else could come into conflict with our survival as a species, resulting in nobody ever having joy again, and a loss of all value-as-ascribed-by-creatures-like-us. If one ruminates on this from the perspective of (2), most people, anyway, find the possibility horrifying, usually far more horrifying than their own death (especially if they've accepted that, yes, they themselves are going to die regardless)."

Indeed, many people do find the prospect horrifying. But the point to remember is that this literally only matters insofar as it affects the goals established by the perspective of these sentient humans, who have a goal of feeling good, and whose mind-bodies tend to produce a negative feeling to that prospect. As soon as we try to move to (3), we are forced to adopt the "perspective" of some purely rhetorical entity that isn't itself a sentient, willful perspective. Which means we don't have a valid perspective from which to define a goal, or any subsequent "oughts".

Which leaves us back with (2). Good feelings are the only known goal system of a sentient being, and therefore the only goals we actually know about, and therefore are the goal that we have to adopt when determining our "oughts".