Get absolute position of vertex in shader

I am trying to clip my object based on an absolute plane or distance, however I am unable to get a value that is not directly tied to the camera position and rotation.

here’s basically what I want to achieve, but it should remain the same when camera moves around it.

To see what happens when the camera rotates, here’s an animated version:

I thought this would be a rather simple thing to do, but shaders are still strange for me.

Here’s some of my experiments with non-working code:

 void vert (inout appdata_full v, out Input o) {
     o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
 }
 
 void surf (Input IN, inout SurfaceOutput o) {
     half4 c = tex2D (_MainTex, IN.uv_MainTex);
     o.Albedo = c.rgb;
     o.Alpha = c.a;
 
     float3 origin = mul(_Object2World, float4(0.0,0.0,0.0,1.0)).xyz;
     float3 cameraDir = mul((float3x3)UNITY_MATRIX_V,float3(0,0,1));
     float3 worldPos = mul(UNITY_MATRIX_T_MV, IN.pos);

     // HERE IS WHAT I CANNOT FIGURE OUT
     // I have tried combining several things such as UNITY_MATRIX_IT_MV and such, to no avail
     //o.Alpha = distance(_WorldSpaceCameraPos + cameraDir -IN.pos - origin, float3(0,0,0)) > 240;
     //o.Alpha = distance(worldPos, float3(0,0,0)) < 120;
     o.Alpha = distance(IN.pos, _WorldSpaceCameraPos) < 240;
 
     o.Albedo = float3(0.5,0.5,0.5);
 
     clip( o.Alpha - 0.5);
 
     o.Albedo.r = IN.normal.x + _Object2World;
     o.Albedo.g = IN.normal.y+sin(IN.normal.z);
     o.Albedo.b = 1-distance(IN.pos.xyz, _WorldSpaceCameraPos)/240;
 }

From what I understand, I would have to revert the projection or view matrix, but I don’t understand how to do that.

Thanks in advance!