Using different shaders based on desktop or mobile?

It may lead to longer load times, but you could switch shaders in Awake based on the platform:

// Your Assets folder should contain a folder called Resources,
// containing a shader called mobileShader
Shader mobileShader = Resources.Load("mobileShader") as Shader;

// Called when a GameObject with this script is loaded
void Awake() 
{
    // Check if we are on mobile
    if (Application.platform == RuntimePlatform.Android || Application.platform == RuntimePlatform.IPhonePlayer)
    {
        // Change just the shader
        GetComponent<Renderer>().material.shader = mobileShader;
    }
}

You can also use shader compile directives in custom shaders to skip over expensive sections, resulting in a simpler look, which only triggers when on a mobile device.

https://docs.unity3d.com/Manual/SL-MultipleProgramVariants.html

I've got a shader I wrote which uses that feature that wraps around a visual effect, the effect can then be disabled in the in-game options menu (and being purely visual, although cool, the game play isn't changed).