Why won't my transparent with shader allow me to change the opacity

I have a shader, which allows me to cast a shadow on a transparent material (which I found here on answers).

It works great; except for one thing : I cna't change the intensity/opacity/blackness of the shadow (anyone of those would make me happy; though I'd prefer opactiy properly on my device.

On the desktop, i found that if I put the albedo value up, that my shadow would become more white, and therefore more transparent. It works a bit on my iOS device, but after about 40% of opacity, it just stops getting any ligter (i.e. I can do albedo 100 and it's the same as albedo 2.

Does anyone have any idea about how to fix this shader on iOS? I've spent days stuck on it..

Here it is

 Shader "FX/Matte Shadow" {
 Properties {
     _Color ("Main Color", Color) = (1,1,1,1)
     _MainTex ("Base (RGB) Trans (A)", 2D) = "white" {}
     _Cutoff ("Alpha cutoff", Range(0,1)) = 0.01
     _ShadowStrength ("Shadow strength", Range(0,5)) = 1
 }
 
 SubShader {
     Tags {"Queue"="AlphaTest" "IgnoreProjector"="True" "RenderType"="TransparentCutout"}
     LOD 200
     Blend Zero SrcColor
 CGPROGRAM
 #pragma surface surf ShadowOnly alphatest:_Cutoff
 fixed4 _Color;
 float _ShadowStrength;

 struct Input {
     float2 uv_MainTex;
 };

 inline fixed4 LightingShadowOnly (SurfaceOutput s, fixed3 lightDir, fixed atten){
   
     fixed4 c;
     c.rgb = s.Albedo*atten;
     c.a = s.Alpha;
     return c;
 }

 void surf (Input IN, inout SurfaceOutput o) {
     fixed4 c = _Color;
     o.Albedo = c.rgb  * _ShadowStrength;
     o.Alpha = 0;
 }
 ENDCG
 }
 Fallback "Transparent/Cutout/VertexLit"
 }