First I just want to say that I LOVE the aesthetic you've got going on here. Big fan of the low res, limited palette, retro vibe, although I do feel like this could be improved in some areas.
Most notably, the particle effects for enemy deaths don't conform to the same strict colour palette that's present for the other sprites, nor do they keep to the same rigid movement style that enemies do. There are some tools available for Unity that come help overcome these issues, I can send you a link to them if that's something you're interested in.
Some other QoL features would also go a long way such as the inclusion of a pause mode, controller support and firing with the directional keys, which was my first instinct considering this is a twin-stick shooter.
Also a huge fan of the enemy spawners and the animation of each new enemy being built piece by piece, an excellent little feature.