How to clean on RenderTexture to transparent with brush

Hi Everyone, (this is the question I posted here)

[done]1, I created RenderTexture, initial it in color(1, 1, 1, 0) with shader and attached to Plane object. And one checker pattern background behind it.

[done]2, Use Graphics.DrawTexture() to draw my brush texture (Tga format with alpha channel) with Alpha blending shader on it.("Blend SrcAlpha OneMinusSrcAlpha" in shader)(See Image1: the checker pattern is the background layer)

[Problem] 3, How can I clean the drawing back to transparent with my brush with correct shader in Unity?

 Shader "Custom/CleanerShader" {
 Properties {
     // Adds Color field we can modify
     _Color ("Main Color", Color) = (1, 1, 1, 0)        
     _MainTex ("Base (RGB)", 2D) = "white" {}
 }

 SubShader {
     Tags { "RenderType"="Transparent" }
     LOD 100
     
     Pass {
         Lighting Off
         SetTexture [_MainTex] { 
             // Sets our color as the 'constant' variable
             constantColor [_Color]
             
             // Multiplies color (in constant) with texture
             combine constant * texture
         } 
     }
 }}

I’m new to unity shader and know that my shader changed whole texture to clear color, so the result is transparent square, not my brush shape. (See Image2), Any suggestion for it?? Thx