levels feel the same

Additional suggested change:

Make the timer function only as a bonus point generation, not a hard level end timer.

Many players of these kinds of games like to take their time to discover the entire level, point score be damned.

It was only after the very long intro had finished, that I discovered two things:
- there was a way to skip it
- there is actually a decent game behind it

You have several options to improve this:
1. make a big button on the intro screen that says "SKIP INTRO"
2. have the intro be optional (replace it with the "quit game" button that basically does nothing useful)
3. shorten the intro significantly

The game reminds me of Commander Keen / Duke Nukem.
The story there was basically "there is bad guys, you is good guy, go shoot 'em"

Absolutely no need for so much heavy handed fluff. It takes away from the experience.