It was an interesting game. Was definitely fun to play, and has an interesting backstory to the playable character. You've absolutely nailed the dark atmosphere to where I was really , really spooked while walking in the stairwell the second time. And I think that the art through the posters was amazing!
Now the only things I wished there were more of in the game were the story; I found it really interesting and engaging in just the letter alone but wished there was more of it. As to why the player is hallucinating, and why the emphasis on boxes and also why we moved. And also wished there was more answers to who the monster was and why is he around us? I think that it's more than a monster and more of the player's fears from their hallucinations or a metaphor for a dark part of the player's issues, but it's not fully explored in there. If it was, I might have missed the parts, and I apologise for that.
Another thing I wish that had some explanation is what the player is doing when they go into the Rec Centre, and why time skips a lot. I understand you tried to keep the game short, but maybe a little more of a routine and seeing what the mundane tasks would emphasise the spooks and scares a lot more!
Another thing you should add more of in future updates is the scares. Because you absolutely nailed the ones that you did in the game (except the last one, which will be discussed in a bit). The part where you can see the monster in the dark and disappears when the player turns the light on and also when the monster is viewable through the crack of the door was absolutely amazing and was one of the better scares I've seen in many horror games. This goes to show that games do not need jump scares or loud sounds to be scary, and you've done so much more than a lot of horror games just by the ambience, and psychological scares.
But you've done it so well in the parts that were meant to be scary that the parts that were the normal walking time was a little lackluster. I do understand that there can't be scary parts everywhere, but I wished that the stairs had something that would give me a fright or keep me on edge because it was the perfect setting; dark shadows, flashy lights, enough space for a monster, multiple floors. The disappearing painting was a nice touch, though an explanation as to why would make it more enthralling into the whole game.
The last thing that I would suggest maybe tweaking would be the final scare at the end when the monster pops out of the ceiling and drops down on the player. And I personally think that this would be more scary/ meaningful of a scare if there was more backstory and story throughout the game, that would make the monster dropping down on the player an eye opener or making the player think.
Sorry for the huge wall of comment but yeah, it was hoenstly a great game. You have nailed the atmosphere and also the spooks and done them really well. The lighting and background music just went so well together that I ended up sweating bullets while exploring the map.
Good luck on future updates if you do, do them and future projects!