You should totally expand on the abstract horror concept. It's interesting that you kinda mixed the concept of monsters vs. supernatural "things" vs. psychological influences (due to decreasing physiological / mental health of the character). Have you played Darkwood by the way? I think you'd find a lot of resonance in there!
As for the SFX mastering it's kinda tricky because the process is kinda the same as mastering music, but not really, because even after you even out the LUFS levels of all the SFX files you still need to go by ear and correct each specific SFX file (if needed).
I need to play more games like these to understand the Dev's choices of horror music, I've just launched a horror music pack recently (still working on the promo art, very cool stuff coming soon), but I had the impression that it was the atmospheres that were appreciated the most, so I'm not sure if the other music are really useful.
That being said I'm curious to see what do you think of this one:
https://drive.google.com/file/d/1F2x-c5bU87eNaZG-C8FSsYk_e24P8ZJO/view?usp=drive...