>Depressurization
That's the damn word I was looking for and I couldn't find (nor remember) anything on google but airplane related stuff, sorry. Despite it being quite buggy, it is still a pretty decent game and it'll be better if you can fix it. It doesn't add much right now as you have little content in your demo, but let us just say it can be considered a core mechanic even if you don't make use of it in space all the time, don't give up. People do tend to like these kind of realistic mechanics.
I wouldn't say you have any sort of feature creep as it's usually reversed for 'games' like Star Citizen which definitely have feature creep over the course of their scam. You have stealth mechanics being able to interact with one another but with a lack of a finisher(takedown), sound which works on both the stealth side and gunplay/'loud' type of play. Not that I'd say no for you to think about what you want your game to have, but haven't you already implemented(stealth/alarms/space system mechanics) what you wanted the game to have?
>Progress is really slow because I don't have discipline and have low motivation at the moment.
Start exercising, go for a jog or lift but really don't give up because you think you can't handle good mechanics introduced in your game.