It's a good start, however there is no reason not to just keep firing. Some thinks you could improve:
> Upon spawning the bullets parent them to the player object, so that you can't walk in one direction and catch up to your bullets.
> Change the shooting to shoot when you press space (instead of hold down space)
> Allow for diagonal shooting.
> Create a reason for the player to not endlessly shoot (par example limited ammo).

Cool game, the powerups would go to the corners and stay there but overall a nice game

It's a fine little game, although with a lot of flaws.
I like the different kinds of enemies, power-ups, special abilities and achievements. This shows some effort has been put into creating the game.

However, I think your movement speed is too low to comfortably move around (I'd at least double it) and everything is way too small. This makes enemies (who are about the size of your projectile) harder to hit and even if you *do* hit them, they take a lot of shots to finish off.
I think the benchmark generally is that the weakest enemy should go down in 1-3 hits. Otherwise, if you don't use special abilities, you can go through the whole game without killing a single enemy even if try hard. And that makes the gameplay uninteresting.

As for the UI, it's pretty messy (especially when you die – it appears all over the place with some text even being on top of the rest).
No clear colour scheme (colours are more or less random), various border thickness, the default rounded UI checkbox – but other things have sharp corners, no alignment, text going out of their respective buttons, pause button being cut in half (and since the cursor is hidden during gameplay, it's harder to click it, too; and once you unpause the game by clicking, the button remains focused and pressing space pauses the game again), random font-style (some headlines are italic, some all caps, some neither), etc, etc.
The worst offender is probably the health bar which has 6 digit precision and the text gets smaller to fit in. If you're passing a float value to the Text field, you could round it before (Mathf.Round) or do something like textField.text = $"{health:F0}".
You also don't need to write the condition directly in the achievement description. "Gain more than 300 points." sounds better than "Score >= 300".

I think the game could be fine, but these are the things which drag its quality down.