[SOLVED]Billboard sprite shader

Hi guys !

I'm trying to modify the "Sprite - Default" shader (from the builtin_shaders-5.5.0f3) with another shader that I found to make my sprite become billboards.

Currently the billboarding part of the code is operating well (sprites are facing the camera) but I got some weird "disappearing"... Some sprites are blinking or disappearing when I move the camera (even in viewport).

No doubt it comes from the part of code I've implemented but since I'm fairly new in shader programming, I have absolutely no clue on how to solve this.

here's the shader :

 Shader "Custom/Sprite_Billboard" {
     Properties
     {
         [PerRendererData] _MainTex ("Sprite Texture", 2D) = "white" {}
         _Color ("Tint", Color) = (1,1,1,1)
         //_Time ("Time", Float) = 0
         [MaterialToggle] PixelSnap ("Pixel snap", Float) = 0
     }
 
     SubShader
     {
         Tags
         { 
             "Queue"="Transparent"
             "SortingLayer"="Resources_Sprites" 
             "IgnoreProjector"="True" 
             "RenderType"="Transparent" 
             "PreviewType"="Plane"
             "CanUseSpriteAtlas"="True"
         }
 
         Cull Off
         Lighting Off
         ZWrite Off
         Blend One OneMinusSrcAlpha
 
         Pass
         {
         CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             #pragma target 2.0
             #pragma multi_compile _ PIXELSNAP_ON
             #pragma multi_compile _ ETC1_EXTERNAL_ALPHA
             #include "UnityCG.cginc"
 
 //            uniform Float _Time;
 
             struct appdata_t
             {
                 float4 vertex   : POSITION;
                 float4 color    : COLOR;
                 float2 texcoord : TEXCOORD0;
                 UNITY_VERTEX_INPUT_INSTANCE_ID
             };
 
             struct v2f
             {
                 float4 vertex   : SV_POSITION;
                 fixed4 color    : COLOR;
                 float2 texcoord  : TEXCOORD0;
                 UNITY_VERTEX_OUTPUT_STEREO
             };
             
             fixed4 _Color;
 
             v2f vert(appdata_t IN)
             {
                 v2f OUT;
                 UNITY_SETUP_INSTANCE_ID(IN);
                 UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(OUT);
 //                OUT.vertex = UnityObjectToClipPos(IN.vertex);
                 OUT.texcoord = IN.texcoord;
                 OUT.color = IN.color * _Color;
             //    #ifdef PIXELSNAP_ON
 
                 OUT.vertex = mul(UNITY_MATRIX_P, 
                  mul(UNITY_MATRIX_MV, float4(0.0, 0.0, 0.0, 1.0))
                 - float4(IN.vertex.y, IN.vertex.x, 0.0, 0.0)
                  * float4(0.1, 0.1, 1.0, 1.0));
 
 //                OUT.vertex = UnityPixelSnap (OUT.vertex);
             //    #endif
 
                 return OUT;
             }
 
             sampler2D _MainTex;
             sampler2D _AlphaTex;
 
             fixed4 SampleSpriteTexture (float2 uv)
             {
                 fixed4 color = tex2D (_MainTex, uv);
 
 #if ETC1_EXTERNAL_ALPHA
                 // get the color from an external texture (usecase: Alpha support for ETC1 on android)
                 color.a = tex2D (_AlphaTex, uv).r;
 #endif //ETC1_EXTERNAL_ALPHA
 
                 return color;
             }
 
             fixed4 frag(v2f IN) : SV_Target
             {
                 fixed4 c = SampleSpriteTexture (IN.texcoord) * IN.color;
                 c.rgb *= c.a;
                 return c;
             }
         ENDCG
         }
     }
 }

Does anyone have a clue on how I can solve this problem ?