Came here after reading your asking on Reddit.
The game is mysterious and the feeling of horror is good. The style of the images is consistent and the UI and interaction is good.
I would maybe add more images of things like the furniture you inspect. The presence of some SFX and ambient sounds is a bit high. Maybe lower their volume would be better (I’m talking mainly of the rain sound).
About your questions:
whether the premise hooks you: Seems interesting and mysterious enough to want to know more
whether the writing and atmosphere work: Writing (in spanish at least) is very good. Atmospheric, descriptive… Personally I would remove the jokes from the text. They “break” the horror mood.
whether you’d want to keep playing after the demo: Personally I have not much time to play anything xD but If I had the time, maybe I could play it.
whether a full version at around $5 feels reasonable: This is something that could not be evaluated with just this “demo”. Also, the pricing depends on a lot of more factors. My recommendation is that you set the price later, when everything is finished, and you have the best version of the game to know the opinion of players.
and what feels weak, unclear, boring, or underdeveloped.
I already told you my suggenstions. The rest of the game seems solid.