I use Aseprite for pixel art. Most of the art I used was free assets that I just edited though (credited above) aside from the player and bullets.
For the glow effect, I do it in Unity by using Shader Graph (to multiply glowing areas by an HDR color) and enabling Bloom. I also place lights behind stuff to actually, well, emit light.
Brackeys made a great tutorial on the topic on YouTube titled "How to make 2D GLOW in Unity!". I think you use Godot though, so you might need to find a tutorial for that. Pretty sure the concept itself doesn't change much anyway. Good luck!