Love it so far! The environment art is great, and the core loop is pretty fun so far. The character designs are simple but expressive, and the different enemies are easy to tell apart at a glance.
If I were to make some changes, I would "juice" the mechanics some. I've linked an excellent video about that by GMTK (if you haven't heard of GMTK, go binge all of his videos, they are so useful and endlessly entertaining). TLDR little things like camera shake and bass-boosted sound effects go a long way to make the game feel fun.
I'd also add music, but I'm assuming you haven't gotten there yet lol. There's lot of creative commons music out there if you either don't do music or just want some placeholder beats, but anything to fill the silence would be nice.
Finally, can you make R reload the gun, and make a different button restart the game? Making the key something unlikely to be pressed by mistake like F1 would be good if you just plan to use it for debugging, while removing the key entirely and adding a pause menu with a quit button would be a good long-term feature. Plus, having to shoot a bunch of random bullets to reload gets a tad annoying. Keep the auto-reload, but also add manual reloading.
Overall, great stuff! It's fun, it looks good, and I can't wait to see how it improves over time!