I'm jumping into this a bit late, but to the OP:
Objective meaning doesn't seem to be any different from an objective physical perspective: it's such a self-contradiction that it's reduced to a meaningless assemblage of words, an incoherent jumble of bits and pieces of actual concepts. You can't see a room "objectively." Sight is necessarily subjective; it's how a particular body interprets particular stimuli from a particular perspective.
This doesn't bother me. I can look at my living room right now and feel comfortable in my "knowledge" of how it looks, which has very real consequences on me and how I live my life, even knowing that it would look different from another perspective or another body. I don't feel a sense of loss, alienation, or lack of foundation when I consider that my perspective is just one of many possible interpretations or that the objects I see exist not objectively as such, but subjectively through my relationships with everything. My natural tendency to interpret my visual stimuli as light passing through a solid window and solid shades and reflecting off of a glass table with a slight green tint to it doesn't seem cheapened to me because it isn't Objective Truth, but rather a subjective construction based on my interactions with existence.
The fundamental phenomena that make up my existence are no different from the morals, values, desires, and aversions that drive it. They aren't objectively inscribed into the universe, a hard set of rules hidden somewhere for the worthy to discover. The universe could be interpreted differently. I could interpret all of the light I see as objects, or none of it. My table could be a collection of atoms or part of a parallel continuity of particles or space that indiscriminately includes what I call table top and what I call empty air. I could be a homosexual, or just a person who has had sex with men. The highest moral value could be to protect the rights of the individual and to develop oneself to the highest degree, or to selflessly contribute to a flourishing human society, or to selflessly contribute to all life. Or life could be completely undifferentiated from non-life, categories of organic and inorganic redrawn or abandoned entirely.
They're all just interpretations.
Now while this doesn't bother me, I can somewhat empathize with why it might bother others. In your original post you go through a long progression of "whys." Why prize a certain morality? Why prefer pleasure to suffering? Why consider something meaningful? I assume that the real pressing need behind this is to reduce meaning and ethics to logical, undeniable, objective foundation: "Z is undeniably true because the core nature of human existence is A, B-Y follows from A, and thus Z is one of the many ultimate expressions of A."
What I might suggest considering instead is understanding truth not as objective, but as perspectival, an interpretation. This doesn't mean that anything can be true. If we go back to my earlier example, we might propose that matter exists as a starting point (even if we could propose an interpretation of reality where it doesn't). My suggestion is that there is no objective nature or properties to this matter. Just like I can't have an objective perspective of my room, we can't say that a table is objectively hard. Rather, we interpret it as such because that is how we experience it. An incredibly small sub-atomic particle would find my table to be primarily empty space and not hard at all. When I say that if I throw my table at you it will strike with a momentum equal to its mass times its velocity, I'm not speaking to an objectively true nature of the universe. Mass, momentum, objects, time, and distance are all just the kinds of things that organisms like us use to interpret reality.
So, getting back to your original question, why should we hold one view over the other? Say, hypothetically, I propose a system of physics that also predicts what happens when I throw my table at you. Instead of just proposing an object with properties like mass, velocity, and momentum it describes an incredibly-complex field of forces in constant interaction. Instead of [momentum = mass X volume] it uses a tangled series of 30-100 calculations depending on the situation, many of which have exceptions or special rules in certain behaviors. My system of physics is able to predict what you and the table will do just as accurately as Newtonian physics; it just uses a much more garbled, complicated way to do so.
Is there any doubt which science we would use?
Humans do not exist objectively, and as such should not be concerned by the fact that objectivity is not a possible, or even logically-coherent, goal to achieve. Instead, we use what works best for us in our subjective positions. In the case of science, our interests are simple: be able to predict and manipulate what will happen in the natural world. Things like time and space are subjective, but they're what help us usefully communicate what is happening. Positing objects that exist in space and have enduring properties over time is just one metaphysical perspective, but it's a damn-useful one, particularly when we're able to keep our objects and the proposed forces and laws that supposedly rule them simple. We would choose Newtonian physics over my infuriatingly-complex physics not because one is objectively better or truer than the other (as far as "truth" and being able to accurately describe the world, they're on perfectly-even footing), but because the simplicity of Newtonian physics.
Ontological parsimony is an openly-acknowledge value of science not because it is objectively better to propose fewer hypothetical entities like momentum and velocity, but because it's better for humans' subjective purpose of understanding and manipulating the world.
When we apply this principle to something like ethics and meaning, it leads to the conclusion that one should be a psychologist first and a philosopher second. What is good or true isn't so because it is objective, it is so because that's what works best for subjective human ends. We are not some Platonic ideal looking down upon a universe of impure manifestations; we're a very particular phenomena in a very particular perspective. If you want to ask why I should strive to be happy, I could give you explanations about evolutionary biopsychology and how organisms with a strong drive towards behaviors that encourage the ongoing survival of their species will endure over time and have more potential to flourish than organisms without such a drive, but that's largely missing the point. I'm not an objective eye gazing upon the unfolding of humanity; that's impossible. All ideation comes from a subject, and without a limitless god that means that all ideation is necessarily subjective. What I AM is a living person in a very specific set of circumstances, and I perceive happiness as preferable to sadness so I pursue it. I empathize with others, and so I pursue their happiness, too. I use logic to arrive at ways to best implement my values, not as a justification for them, because I understand that at the end of the day a value is a perspective, and thus not something that can ever be objectively true.
To give credit where credit is due, I should mention that much of this stems from my readings of Friedrich Nietzsche, my favorite human psychologist who also dabbled in philosophy and perhaps the most famous person to tackle the issues you bring up. I strongly endorse what he came to call perspectivism, and so I'll end this (far-too-verbose) post with a quotation from him defining it:
"In so far as the word 'knowledge' has any meaning, the world is knowable; but it is interpretable otherwise, it has no meaning behind it, but countless meanings."
