Color To Alpha shader for android not working

I am having ca. 100 rgb textures which I try to render them in Realtime with Alpha channel by reading its pixels and discard some with a target color that will create at those transparency.

Therefore the general file size is dramatically decreased (half of those textures filesize). I have found a shader that suits my needs and this works with no issues on PC. However this does not work when testing on android.

 Shader "Custom/ColorToAlpha" {
  Properties {
      _Color ("Color", Color) = (1,1,1,1)
      _TargetColor ("Transparent Color", Color) = (0,0,0,1)
      _Threshold ("Threshhold", Float) = 0.1
      _MainTex ("Albedo (RGB)", 2D) = "white" {}
  }
  SubShader {
      Tags { "Queue"="Transparent" "RenderType"="Transparent" }
      LOD 200
      Cull Off
      CGPROGRAM
      #pragma surface surf Lambert //alpha
 
      sampler2D _MainTex;
 
      struct Input {
          float2 uv_MainTex;
      };
 
      fixed4 _Color;
      fixed4 _TargetColor;
      half _Threshold;
 
      void surf (Input IN, inout SurfaceOutput o) {
          // Read color from the texture
          half4 c = tex2D (_MainTex, IN.uv_MainTex);
          
          // Output color will be the texture color * the vertex color
          half4 output_col = c * _Color;
          
          //calculate the difference between the texture color and the transparent color
          half3 transparent_diff = c.xyz - _TargetColor.xyz;
          half transparent_diff_squared = dot(transparent_diff,transparent_diff);
          
          //if color is too close to the transparent one, discard it.
          if(transparent_diff_squared < _Threshold)
            discard;
          
          //output albedo and alpha just like a normal shader
          o.Albedo = output_col.rgb;
          o.Alpha = output_col.a;
      }
      ENDCG
  } 
  FallBack "Diffuse"
 }