I love the chiptune sound of this song, I could so see this appearing in a low-bit horror game!
At the beginning I am not really getting post-boss battle vibes, it really feels much more like a theme that would be played as you explore a decrepit house or something like that. But as we get more towards the middle I am definitely seeing that reflection on the boss battle theme more, especially with the focus on percussion sounds and a building tension in the lead instrument. I really love this part, it's great!
As a final comment, one thing I do notice is that this song runs pretty long for this competition. Instead of a longer 5 minute composition, I think it would be great if you took the middle→end parts and turned that into something shorter. That would give you more time to really hammer out all the kinks and really get a masterpiece.
Great job, this is awesome!