Going into OW, I knew that there was a time loop and the sun kept exploding. That was it. The first 1/3 of my playthrough was in line with my limited expectations. There's space, planets, and an ancient precursor race that liked science maybe a bit too much. Also something about an Eye of the universe and quantum mechanics. I'll probably be relevant later.
Then all the scattered pieces of information started coming together, and I learned more about the reasons why everything is happening the way it did. A neat space adventure game gradually morphed into an exploration of the uncaring nature of the universe, the value of impermanent life, making the most of the time you have left, and finding meaning where no intrinsic meaning exists. A lot of the dialogue hits surprisingly hard, and the Nomai went from being this race of mad scientists who I was kinda laughing at to feeling depressed over what they experienced. I ignored the spoiler box discussion on the Sun Station revelation earlier in this topic, but for me it was the moment where everything truly clicked. This wasn't a game about saving the universe - this universe, at least. That combined with the Interloper revelation is what drove the themes home for me. The Nomai weren't silly mad scientists who blew themselves up (...most of the time). They were doing their best, making actual progress - and then a toxic gas asteroid came and killed them all, and there was nothing they could have done about it. They were just as helpless as the Hearthiens who happened to be born right when the entire universe is turning off the lights.
The DLC also added a lot to the story. Not so much learning that the Owelk's blocked the Eye's signal - that's a neat detail, but it's the themes that got me. When the Nomai teleported into Dark Bramble and got scattered and broken, they picked up the pieces and kept going on. They didn't let that tragedy define them, and you can see in their messages that they never lost their passion for life. In contrast, when the Owelk's learned that the Eye of the Universe meant a death and rebirth of everything, they couldn't handle it. Rather than own up to their mistake of strip-mining their planet, they put the Eye on block and retreated into a virtual fantasy world, letting their bodies waste away. They're all dead by the time you arrive, but unlike the Nomai, they did this to themselves. All that just makes the final 'conversation' with the prisoner, where you show him that not everything was in vain, that much more impactful. The one narrative note OW was missing was an emotionally resonant conversation with a living person, and the DLC delivered.
tl;dr - I think this one quote sums up the core themes of the game for me.
SOLANUM: As a child, I considered such unknowns sinister. Now, though, I understand they bear no ill will. The universe is, and we are. I am ready.
Final side note: the quantum stuff in Outer Wilds is all cool as shit. I don't necessarily want other games to repeat it, as it would lose its luster that way, but the Quantum Moon and Eye of the World were probably the most memorable parts of the game for me.