Thanks for your feedback! You are right, since it's my first jam, I made the mistake of making the game take too long to progress (the idea was about 60 minutes).
I added the O₂ mechanic to give you a little bit of stress, but you can upgrade the capacity. It's also intentional that you have to return; it's a bit of brain training, like a memory game! ;)
But a minimap is a great idea for a buyable upgrade! As the cave scales up over time, it might lose some of the fun if it gets too big to remember. It would also discourage players from exploring deeper, so a map would definitely help...