Let me reiterate your objections to make sure I'm understanding them correctly. I'm taking your first objection to be against my premise 1 (the beauty of the universe is improbable under atheism), and it looks like you're arguing this way:
Beauty is not improbable under atheism since it provides a hit of dopamine, which improves mental health, which helps us survive.
It seems like here you're talking about the experience/appreciation of beauty in the world, rather than the beauty itself of the world, which of course makes sense since you argue later that there is no objective quality of "beauty". So I think you're arguing that it isn't improbable under atheism that people would experience beauty. My response then is that you've changed my premise and argued against something I didn’t claim (which means I probably ought to have been more clear when I wrote it).
Maybe if I can argue better that beauty is at least partly objective, then I can respond to both your objections. Your second objection is that beauty is entirely subjective. I'll regiment your argument like this:
1- If tastes in art vary widely, then beauty is entirely subjective.
2- Tastes in art do vary widely.
3- Therefore, beauty is entirely subjective (1,2 MP)
I have a couple objections to premise 1. First of all, I think it's clear that opinions can vary without changing the fact of the matter. For example, jurors might disagree on whether a defendant is guilty or innocent, but this doesn’t change whether or not the defendant actually committed the act. Similarly, maybe people can disagree on whether or not a piece of art is beautiful and that disagreement would have no bearing on whether it’s truly beautiful or not. My other objection is that it seems like most postmodern art isn’t art because it’s beautiful. Beauty doesn’t seem to me to be a necessary condition for something to be art (lots of metal songs can hardly be described as beautiful and yet qualify as art, also Munch’s The Scream and Donatello’s Penitent Magdalene are art but aren’t beautiful). So I don’t think that differing tastes in art prove beauty is subjective.
As far as arguing that beauty is at least partly objective, I’ll stick with my sunset example. It does seems like most people are in agreement that a striking sunset is beautiful. If someone denied its beauty, it seems like I could confidently tell them they’re wrong and have faulty aesthetic senses, so to speak. So it seems to me like there is something about the beauty of the sunset that is objective. My argument looks something like this:
1- If almost all people agree that something is beautiful, then it is at least very probable that there is at least some objective beauty.
2- Almost all people agree striking sunsets are beautiful.
3-Therefore, it is at least very probable that there is at least some objective beauty. (1,2 MP)
But I don’t want to distract from your point that our appreciating beauty could have evolved in an atheistic universe, though, because I think there’s something important there. I’ll list a couple of quick objections to the idea that we evolved our aesthetic sense and see what you think of them:
1- Other animals don’t seem to appreciate beauty. Why is it that only humans do? If appreciating beauty is an advantage for survival, why don’t we see it in other animals?
2- Oftentimes dangerous things in nature are beautiful, like the markings on big cats, lightning, the colors on poisonous tree frogs and blue ringed octopuses, the ocean, fire, etc… So it doesn’t seem like appreciation of beauty is so great for survival after all.
Let me know what you think of all that!