Unity HDRP custom UI shader version that works for both text + images for me. Random trial + error on the blend mode. Not sure which other things are broken now...

Custom_UIAfterPP.shader

// Unity built-in shader source. Copyright (c) 2016 Unity Technologies. MIT license (see license.txt)

Shader "Custom/UI-Default-AfterPP (Images)"// Unity built-in shader source. Copyright (c) 2016 Unity Technologies. MIT license (see license.txt)
{
	Properties
	{
		[PerRendererData] _MainTex("Sprite Texture", 2D) = "white" {}
		_Color("Tint", Color) = (1,1,1,1)

		_StencilComp("Stencil Comparison", Float) = 8
		_Stencil("Stencil ID", Float) = 0
		_StencilOp("Stencil Operation", Float) = 0
		_StencilWriteMask("Stencil Write Mask", Float) = 255
		_StencilReadMask("Stencil Read Mask", Float) = 255

		_ColorMask("Color Mask", Float) = 15

		[Toggle(UNITY_UI_ALPHACLIP)] _UseUIAlphaClip("Use Alpha Clip", Float) = 0
	}

		SubShader
	{
			Tags
		{
			"LIGHTMODE" = "ForwardOnly"
			"RenderType" = "HDUnlitShader"
			"RenderPipeline" = "HDRenderPipeline"
			"Queue" = "Transparent+700" 
			"IgnoreProjector" = "True"
			"PreviewType" = "Plane"
			"CanUseSpriteAtlas" = "True"
		}

		Stencil
		{
			Ref[_Stencil]
			Comp[_StencilComp]
			Pass[_StencilOp]
			ReadMask[_StencilReadMask]
			WriteMask[_StencilWriteMask] 
		}

		Cull Off
		Lighting Off
		ZWrite Off
		ZTest [unity_GUIZTestMode]
		/// this works with everything, kinda
		Blend SrcAlpha OneMinusSrcAlpha, Zero OneMinusSrcAlpha
		/// this works with RawImages and Images
		// Blend SrcAlpha OneMinusSrcAlpha
		/// this works with UI Text but breaks RawImages and Images (in different ways)
		// Blend One OneMinusSrcAlpha, Zero OneMinusSrcAlpha
		ColorMask[_ColorMask]

		Pass
	{

		Name "ForwardOnly"
		Tags
		{
			"LIGHTMODE" = "ForwardOnly"
			"RenderType" = "HDUnlitShader"
			"RenderPipeline" = "HDRenderPipeline"
			"Queue" = "Transparent+700"
			"IgnoreProjector" = "True"
			"PreviewType" = "Plane"
			"CanUseSpriteAtlas" = "True"
		}

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

#include "UnityCG.cginc"
#include "UnityUI.cginc"

#pragma multi_compile_local _ UNITY_UI_CLIP_RECT
#pragma multi_compile_local _ UNITY_UI_ALPHACLIP

		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;
		float4 worldPosition : TEXCOORD1;
		UNITY_VERTEX_OUTPUT_STEREO
	};

	sampler2D _MainTex;
	fixed4 _Color;
	fixed4 _TextureSampleAdd;
	float4 _ClipRect;
	float4 _MainTex_ST;

	v2f vert(appdata_t v)
	{
		v2f OUT;
		UNITY_SETUP_INSTANCE_ID(v);
		UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(OUT);
		OUT.worldPosition = v.vertex;
		OUT.vertex = UnityObjectToClipPos(OUT.worldPosition);

		OUT.texcoord = TRANSFORM_TEX(v.texcoord, _MainTex);

		OUT.color = v.color * _Color;
		return OUT;
	}

	fixed4 frag(v2f IN) : SV_Target
	{
		half4 color = (tex2D(_MainTex, IN.texcoord) + _TextureSampleAdd) * IN.color;

#ifdef UNITY_UI_CLIP_RECT
		color.a *= UnityGet2DClipping(IN.worldPosition.xy, _ClipRect);
#endif

#ifdef UNITY_UI_ALPHACLIP
		clip(color.a - 0.001);
#endif

		return color;
	}
		ENDCG
	}
	}
}