Question on using stencil buffer + trail renderers that fade out

Unity Engine
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1

New to shaders so apologies if anything is off in my description of the problem.

I have a shader that is basically just a copy of Unity’s Legacy Shaders/Particles/Alpha Blended shader, modified to always write 1 to the stencil buffer. The shader is then used in a material plugged into a particle system.

In a separate shader, used on a box in this case, I am reading this value from the buffer, allowing me to render the box when the trail passes over it. The effect works, but the shape stays brightly lit regardless of the trail fading out (see gif below). In my limited understanding of shaders I get why this is the case, but I am a bit stumped on a solution here.

Does anyone have any thoughts on how I could solve this? I am even open to solutions that don’t involve shaders; I just need the trail to change color as it passes over particular shapes.

Shader for the box

Shader "Custom/StencilRead"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
        _Color ("Tint", Color) = (1,1,1,1)
    }
    SubShader
    {
        Tags { "RenderType"="Opaque" "Queue"="Transparent" }

        Pass
        {
            ZWrite Off

            Stencil
            {
                Ref 1
                Comp Equal
            }
            
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag

            // #include "UnityCG.cginc"

            struct appdata
            {
                float4 vertex : POSITION;
                fixed4 color : COLOR;
                float2 texcoord  : TEXCOORD0;
            };

            struct v2f
            {
                float4 vertex : SV_POSITION;
                float4 color : COLOR;
                float2 texcoord : TEXCORD0;
            };

            sampler2D _MainTex;
            float4 _Color;

            v2f vert (appdata IN)
            {
                v2f OUT;
                OUT.vertex = UnityObjectToClipPos(IN.vertex);
                OUT.texcoord = IN.texcoord;
                OUT.color = IN.color;
                return OUT;
            }

            fixed4 frag (v2f i) : SV_Target
            {
                fixed4 texColor = tex2D(_MainTex, i.texcoord) * i.color;
                return texColor;
            }
            ENDCG
        }
    }
}

Shader for the particle system (copy of Legacy Shaders/Particles/Alpha Blended but writes to Stencil buffer)

Shader "Custom/StencilWrite"
{

    Properties {
        _MainTex ("Particle Texture", 2D) = "white" {}
        _InvFade ("Soft Particles Factor", Range(0.01,3.0)) = 1.0
    }

    Category {
        Tags { "Queue"="Geometry-1" "RenderType"="Transparent" }
        Blend One OneMinusSrcAlpha
        ColorMask RGB
        Cull Off Lighting Off ZWrite Off

        SubShader {

            Pass {

                Stencil
                {
                    Ref 1
                    Comp Always
                    Pass Replace
                }

                CGPROGRAM
                #pragma vertex vert
                #pragma fragment frag
                #pragma target 2.0
                #pragma multi_compile_particles

                #include "UnityCG.cginc"

                sampler2D _MainTex;
                fixed4 _TintColor;

                struct appdata_t {
                    float4 vertex : POSITION;
                    fixed4 color : COLOR;
                    float2 texcoord : TEXCOORD0;
                    UNITY_VERTEX_INPUT_INSTANCE_ID
                };

                struct v2f {
                    float4 vertex : SV_POSITION;
                    fixed4 color : COLOR;
                    float2 texcoord : TEXCOORD0;
                    #ifdef SOFTPARTICLES_ON
                    float4 projPos : TEXCOORD1;
                    #endif
                    UNITY_VERTEX_OUTPUT_STEREO
                };

                float4 _MainTex_ST;

                v2f vert (appdata_t v)
                {
                    v2f o;
                    UNITY_SETUP_INSTANCE_ID(v);
                    UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(o);
                    o.vertex = UnityObjectToClipPos(v.vertex);
                    #ifdef SOFTPARTICLES_ON
                    o.projPos = ComputeScreenPos (o.vertex);
                    COMPUTE_EYEDEPTH(o.projPos.z);
                    #endif
                    o.color = v.color;
                    o.texcoord = TRANSFORM_TEX(v.texcoord,_MainTex);
                    return o;
                }

                UNITY_DECLARE_DEPTH_TEXTURE(_CameraDepthTexture);
                float _InvFade;

                fixed4 frag (v2f i) : SV_Target
                {
                    #ifdef SOFTPARTICLES_ON
                    float sceneZ = LinearEyeDepth (SAMPLE_DEPTH_TEXTURE_PROJ(_CameraDepthTexture, UNITY_PROJ_COORD(i.projPos)));
                    float partZ = i.projPos.z;
                    float fade = saturate (_InvFade * (sceneZ-partZ));
                    i.color.a *= fade;
                    #endif

                    return i.color * tex2D(_MainTex, i.texcoord) * i.color.a;
                }
                ENDCG
            }
        }
    }
}

particles