Time for a quick update.
Made some progress:
Sound, as mentioned in my last post, is implemented.
Post-processing has been implemented. I fear that to achieve the exact atmosphere I aimed for, I would have had to use HDRP, but it's okay.
I have implemented half of the UI, including the death screen and different status messages, etc.
I have a prototype for a main menu.
Currently, I am working on the main mechanics, such as decreasing the oxygen tank, death, and the main events of the game.
If everything goes as planned, I will be able to make the first build next week :)