cousin_it and Vladimir_Nesov's replies are good answers; at the risk of being redundant, I'll take this point by point.

to some alien species we might encounter or AI we might build, our values would be as meaningless as the values of insects are to us.

The above is factually correct.

humanity’s values have no cosmic significance or absolute validity in our vast cosmos

The phrases "cosmic significance" and "absolute validity" are confused notions. They don't actually refer to anything in the world. For more on this kind of thing you will want to read the Reductionism Sequence.

all our creations and efforts are ultimately futile in a universe of increasing entropy and astrophysical annihilation

Our efforts would be "ultimately futile" if we were doomed to never achieve our goals, to never satisfy any of our values. If the only things we valued were things like "living for an infinite amount of time", then yes, the heat death of the universe would make all our efforts futile. But if we value things that only require finite resources, like "getting a good night's sleep tonight", then no, our efforts are not a priori futile.

Only egotism exists.

Egotism is an idea, not a thing, so it's meaningless to say that it exists or doesn't exist. You could say "Only egoists exist", but that would be false. You could also say "In the limit of perfect information and perfect rationality, all humans would be egoists", and I believe that's also false. Certainly nothing you've said implies that it's true.

The Metaethics Sequence directly addresses and dissolves the idea that everything seems to be meaningless because there is no objective, universally compelling morality. But the Reductionism Sequence should be read first.

The standard reply here is that duh, values are a property of agents. I'm allowed to have values of my own and strive for things, even if the huge burning blobs of hydrogen in the sky don't share the same goals as me. The prospect of increasing entropy and astrophysical annihilation isn't enough to make me melt and die right now. Obligatory quote from HP:MOR:

"There is no justice in the laws of Nature, Headmaster, no term for fairness in the equations of motion. The universe is neither evil, nor good, it simply does not care. The stars don't care, or the Sun, or the sky. But they don't have to! We care! There is light in the world, and it is us!"

(I'm new here and don't have enough karma to create a thread, so I am posting this question here. Apologies in advance if this is inappropriate.)

Here is a topic I haven’t seen discussed on this forum: the philosophy of “Cosmicism”. If you’re not familiar with it check Wikipedia, but the quick summary is that it’s the philosophy invented by H. P. Lovecraft which posits that humanity’s values have no cosmic significance or absolute validity in our vast cosmos; to some alien species we might encounter or AI we might build, our values would be as meaningless as the values of insects are to us. Furthermore, all our creations and efforts are ultimately futile in a universe of increasing entropy and astrophysical annihilation. Lovecraft’s conclusion is: “good, evil, morality, feelings? Pure 'Victorian fictions'. Only egotism exists."

Personally I find this point of view difficult to refute – it seems as close to the truth about “life, the universe and everything” as one can have and remain consistent with our current understanding of the universe. At the same time, such a philosophy is rather frightening in that a world of egomaniacal cosmicists who consider human values to be meaningless would be seem to be highly unstable and insane.

I don’t claim to be an exceptionally rational person, so I’m asking the rationalists of this forum: what is your response to Cosmicism?

Online poker is the most brutal rationality test I know of. As a one time “semi-professional” player, I experienced things which really strained my capacity to believe in a random universe. It would be amusing to watch someone like Eliezer Yudkowsky play poker; I can easily imagine him becoming an emotional, superstitious nut throwing keyboards against the wall like almost everyone else who plays poker long enough!