A bit too difficult for my tastes, but I can respect your choice to make it harder given the atmosphere you're trying to create. Would be more accessible if the difficulty was more customizable. I really like the artwork: it shows "explorer in a vast, unfamiliar, hostile world" so well. I also wish the character moved faster, even if you have to adjust other things (e.g., speed of oxygen loss) to compensate.
The text was tiny!!! I think it's because you used the default font for Godot, which is tiny, barely readable text. No idea why Godot thinks that's an acceptable default.