Stylish right from the title screen, which sets the tone perfectly. One area that could be pushed further is animation – the title cards feel a little jarring with no transition to or from them, for instance. You also kind of notice the fact that the sprite doesn't have a ton of expressions; the heaviness of the subject matter and the extreme emotions the story invokes aren't really visible, and it doesn't necessarily feel purposefully dissonant. But apart from that, the art & the UI are polished and pretty to look at.
Writing is nice as well! The characters feel precisely defined enough for the small scope, with bits and pieces of history but not so much backstory the story drowns in exposition. There's a lot of sharp imagery (the remnants of a drink in a glass being compared to blood is a particularly memorable one), and the story only ever feels appropriately claustrophobic, maintaining a forward momentum despite its place in the "nothing happens" genre.
Whether the ending gets too fanciful is debatable. I did not hate it, but it maybe felt like it lost the grounded tone of the preceding material without really reaching apocalyptic heights to match it in effectiveness?
Finally, as for the jam theme, I do see it there, but I also wonder if the game could have just gone with the Sun going supernova to wrestle with the idea a little more explicitly. If I was just reading this with no context, it would be pretty difficult to guess that "expanding" was the prompt. That being said, I enjoy what the game does with the concept overall, so don't listen to me.