Okay, I agree with practically everything you've said, except that here you're not even being entirely consistent, though maybe it's too subtle of a way to notice.
But first, the parts that we actually disagree about, which involves bugs, children, evolutionary constraints, and the naturalistic fallacy.
Starting at the end: as Hume famously pointed out, that something is the case is a different statement than that it ought to be. Now, I agree that we have to operate within the constraints of our behavioral control system, wherein positive affect--feeling good--plays a very large role in determining our behavior. So it's also pointless to deliver ought-statements that say that we ought to do something we can't. And it's not much use to take the perspective of Christianity: you ought to do this, but it's impossible, so believe in Jesus-who-was-perfect and your sin is forgiven.
However, as you point out yourself, we do have considerable flexibility to sculpt our interpretation, including our positive and negative affect, on the basis of how we view things. In many cases, it is simply the result of attention. For instance, if I point out to a young child that they shouldn't hit Billy because it hurts him and he doesn't like that, and they too do not like being hurt, I am trying to draw the child's attention to the feelings of empathy and reciprocity that I hope that they have. If they're old enough, and not a sociopath, this will often work. Now the action that they enjoyed before (either out of positive emotion: it's fun to see! or alleviation or reaction to negative: Billy is playing with the toy I want to play with, so frustrating!) will, hopefully, be reinterpreted as a negative. If the child is sociopathic (and, honestly, empathy takes a while to develop, so a lot of kids effectively are), that might not work at all; instead, they might just go, "Oh, I got in trouble, that is bad-for-me, I don't like what is bad for me. I should only hit Billy when nobody is watching, because I want what is good for me and it's good to hit Billy but worse to get caught."
Of course, it can also be a mix. But it is the internalization of the empathic attention which delivers a pro-social behavioral pattern that we rely on to live together in harmony.
It would be an instance of the naturalistic fallacy to say that because the child wanted to hit Billy, they should have hit Billy. But it is no less an instance of the naturalistic fallacy to say that because anyone wants to do anything that therefore they should.
And that's what your argument boils down to.
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.
What you have done is identify the major currency of our behavioral control system and say: morality is getting rich (together) in this currency!
But why is this currency even a currency? What is it for? Clearly, if it were placed there by an evil god in order to cause suffering, we'd be inclined to at least say, "Uh, wait, let's think about this."
So we use all our other intellectual capacities to investigate the problem, and take advantage of the fact that we live in a time in history after we've discovered how to do science, and recognize: this whole system exists because of its adaptive value. (The whole system, not just the feeling good and bad, but also the executive control and attention.)
The only reason we have "ought" is because of the repeated ratcheting of accident-and-selection that fixes traits that have adaptive value. "Ought" is. It just is; it's grounded not in an idea but in a physical mechanism that we happened to evolve. Weird, but all the evidence points that way.
But now when we go back to evaluating what the wellspring should be from which we draw morality, we have at least three choices.
(1) Our moral intuitions: we take our morality as a given, and reason from there.
(2) Our behavioral control: we take our whole behavioral reward system as a given, and reason from there, overriding the moral intuition slice of our behavioral control system as needed based on a broader perspective of how we as creatures are rewarded. (Much of the time the answer will be: moral intuitions got it right; no point fighting it.)
(3) Our evolutionary constraints: we take our role as players in the life process as a given, and reason from there, overriding a naive interpretation of our behavioral control system as needed based on a broader perspective of how we as social primates with complex culture perpetuate ourselves. (Much of the time the answer will be: behavioral reward system got it right; no point fighting it.)
The choice is to some extent arbitrary; it's not like the universe cares whether we feel moral, whether we feel happy, or whether we go extinct.
There is a sizable danger, though, of going with (1), and we can even use (1) to motivate widening our perspective: if we don't take our entire behavioral motivation system into account, our moral intuitions might--because we're not mindful--lead us into a situation with profound and enduring suffering or lack-of-joy. Our sense of empathy may recoil profoundly at that possibility, motivating us not just to keep that in mind, but to abandon moral intuitions as potentially inadequate to even do their own job. We had better consider all of our joys and fears and everything. We can't use this to tell ourselves to engage in things that result in such profound moral revulsion that we can't even do them, but of course we wouldn't: that's part of the analysis of our whole behavioral reward suite.
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). And thus, even from the perspective of (2) this should motivate us to abandon our behavioral control system as potentially inadequate to even do its own job. We had better consider the existence of humanity (and/or life; there are no natural bounds here, though species is a pretty natural demarcation). We can't use this to tell ourselves to engage in things that result in such profound feelings of pain or loss of happiness that we can't even do them, but of course we wouldn't: that's part of the analysis of the evolutionary constraints on keeping our kind going.
So, going back to the moth: we see the moth bopping into the light and we know that this isn't what its feelings of joy are for. It's a trick, a horrible trick, that makes it feel good and die for nothing. So, if you're like me, you catch the moth and toss it outside where it has an opportunity to use its attraction to lights at night to do what the instinct is for: find a mate.
But humans aren't like moths.
Worker ants never reproduce, but it is through their labor that ants continue to exist. Being a part of a society without reproducing yourself is a perfectly viable way to keep society and species (and those of your genes shared with other members of society) going.
So it's perfectly fine for individuals to not have children. Indeed, this explains why we find it incredibly noble for people to sacrifice themselves for their fellows: it isn't noble because they, like everyone, did what they felt like. It's noble because they felt the importance of existence overall in a way that superseded their own personal drives.
However, if nobody is having children, this is not perfectly fine. This would be a crisis of the most profound importance. We don't have such a crisis; having some shrinking populations in a world of eight billion people that are overtaxing the planet as a whole is not remotely an existential crisis.
But it could be.
The point of morality is to allow us to interact in a way that works with our sense of happiness and sadness. The point of happiness and sadness is to allow us to behave in a way that perpetuates us. And that's it; it doesn't go any farther than that.
Stopping at the level of happiness and sadness is unfounded; even though everything is ultimately unfounded in a strong sense, to the extent that there is something founded, it's the evolutionary level not the I-feel-this level that has given us "ought" to begin with, and which is the deepest justification available for anything we do.
Almost always, the answer is just: to safeguard our continued existence, the best solutions involve people being safe and happy so (3) reduces to (2) in all but exceptional situations. And very often, likewise, (2) reduces to (1)--our moral intuitions are decent much of the time. But not always, and when you're thinking in a philosophical vein, the "not always" part of it is important.