I like the mechanic of being given a random selection of upgrades as you defeat enemies - It makes each playful through a little different as I would prioritise collecting different sets of upgrades based on the initial three upgrades I see.
An improvement that I would make would be to remove the damage the ship takes when bumping into the walls, as the small confines makes it hard to manoeuvre around objects and bullets. Especially as more enemies start spawning in. Alternatively, increase the size of the rooms, and the area that the player can see. This would allow the player to plan where to move to when confronting enemies, and allow the player a chance to avoid being dogpiled by hoards of enemies through clever manoeuvring.
The ship's movements does feel a little sluggish, but feels justifiable as it is in space. I like to think that ship's performance is due to the planet being destroyed and that the ship was partially damaged during this process.
There is one thing that I'm not certain on: is it supposed to be an endless game? The game's description implies that it's a last stand and the ship's assault is Atliz's last act of spite, but the level layout gives the impression that there's something that's supposed to be explored followed by an actual endgame i.e. escape the debris, make your way to the enemy planet, and destroy the enemy's planet to enact vengeance.
This leads to other potential improvements:
1) if it is supposed to be an endless game, the game could instead take place in a large area with the enemies and hazards, similar to Geometry Wars. This would also give the player more room for moving around, which would be useful as your game is similar to a bullet hell game.
2) if there is supposed to be an endgame, then include different level layouts to indicate a sense of story progression. For example, part one is the current layout where you escape destroyed planet's debris; part two is predominantly space and asteroids as you travel to the enemy planet; final part is the enemy planet's defences and eventually destroy the planet.
I also like the artstyle. The enemy sprites were distinct, so it was easier to quickly determine what enemies I was facing, remember their respective abilities and then prioritise which ones I should handle first.
Excellent work.