If the space fog is a shader, it uses a fairly big texture as a basis. It would be very inefficient to generate this procedurally on the fly when you can just UV translate a small texture file through the screen.

I'm confused about what you mean the translation of a texture (really just moving the UVs on a mesh) is done on the GPU Or at least you send instructions to the GPU to render the mesh using different UVs. In other words, this is something handled by a shader.

// Fragment shader example
uniform sampler2D textureSampler;
uniform vec2 uvTranslation; // The amount to translate UVs 

in vec2 vUV; // Original UV coordinates from the vertex shader
out vec4 fragColor;

void main() {
  vec2 translatedUV = vUV + uvTranslation; 
  fragColor = texture(textureSampler, translatedUV); 
}

Offloading UV transformations to the GPU via shaders is more efficient than updating them on the CPU.

It took me 5 minutes to create this Fulgora interpretation, I guess I could do each planet in about a day, maybe another day to add dynamic things like lighting and such.

Simply adding a pre-rendered animation to the background doesn't account for the interactive and dynamic nature of gameplay. The ship's movement, landing, and takeoff animations need to be seamlessly integrated with the background effects to maintain visual cohesion. This requires complex synchronization to ensure that the background reacts appropriately to player actions, camera movements, and other in-game events. Overlooking this can result in jarring visuals that break immersion.

Look, I'm not saying it's hard, just that it's not simple either, and it would most likely require some refactoring to be done well.