Great job getting this working in PyGame, that's not a small task. I really like the 2000s military aesthetic you've gone for.
I ran into a bug where dying once seemed to lock me out of progressing further, so I wasn't able to see much beyond that point. definitely worth a look, since it's probably stopping other players from experiencing more of the game too.
A couple of small things that could help: I wasn't sure of the controls at first, so a short on-screen quip (e.g. "mouse to move, click to shoot") would help players get going faster. Also, the "Get Ready!" text felt a bit goofy compared to the more serious military tone elsewhere. Might be worth swapping for something that matches the vibe you're going for.