I like the art style. It's cute. I also like that the modifications at the end of each level are not all simple upgrades but tradeoffs. However, it might be nice to have a couple actual upgrades that don't have any downsides.
You do have the shield, but after the first, the later shield upgrades don't do anything.
Another aspect you could have as a trade off is the bullet range. I often felt like my range was more limited than I wanted and would be willing to trade something for more range.