I think you’ve got something fairly good going here - it just needs a bit of tweaking.
First, yes, the levels are a bit too long.
Another thing is that certain enemies are a bit unfair - especially the ones that fire fast blue bullets. If the player isn’t expecting it, those can prove instantly fatal with no chance to dodge. I would suggest that you do one of the following:
- if the enemy is going to fire a fast bullet, make them play a “charging” animation (blue particles?) just before so the player has some warning, or
- make the bullet start slow and accelerate.
The weapons also need balancing, it seems. Red is a bit overpowered, while green and blue seem a bit underpowered. (btw, in the applet version, the green weapon doesn’t have a sound.)
Scoring system would be more interesting if you added bonuses for doing interesting stuff.
Finally, THE PLAYER’S HITBOX IS HUGE! In practically all commercial shooters these days, the player’s hitbox is smaller than their sprite. About 80% of the time, the hitbox is miniscule - say, about the size of the plane’s cockpit. IMO it would help to decrease the size of the hitbox - THEN you could add more bullets without worrying about unbalancing the game. As a start, try making the hitbox cover only the centre portion of the plane - not the nose or wings.