Hey nice game! Out of the 5 games I've rated so far this one is definitely the best (Presentation, completeness, fun). I do have some problems with the game though.
First of all the concept of the game is basically just a regular bullet hell game but with oxygen mechanics added. Honestly for a large part of the game it just feels like a regular bullet hell, I got to round 5 on my first run without having to understand or think about the oxygen mechanic at all, then I died because I didn't sacrifice anything because I didn't entirely understand the mechanics (I did not read the instructions). Same thing on my second run, I got to round 5 then sacrificed 1 ship and some crew and ended up barely winning.
So the sacrificing mechanic should imo be more prevalent, since it's basically the main selling point.
I think there are 2 main ways to solve this problem that I can think of and those are: 1. Make the game harder so the average player has to sacrifice more to win. 2. Make it so that you lose and gain oxygen more quickly.
I haven't thoroughly thought about how to balance this, but I feel like option 2 would probably make for a more unique/interesting game, it means the player isn't only in danger after a few rounds and you could have more quick thinking with sacrificing during actual gameplay instead of only between rounds which could be fun.
Also for this kind of mechanic you basically do the reverse Mario Kart game balancing (not sure what the technical game designy term for this is) where you buff players for doing well and nerf players for doing poorly, so doing slightly worse early in the game might mean that you do significantly worse later because you don't have as much fire rate or as many ships to help you. Which might not work very well in a linear type of game but could maybe work in an endless game? Not sure honestly, just something to keep in mind if you ever continue with this game.
And a very small complaint but still wanted to mention it, I don't like that P is continue, I have my left hand on wasd and my right hand on the mouse, neither of those can quickly go to P!
This comment ended up being a lot longer than expected :p