Smoothing pixels via Shader / Tree Leaf Shader

Hi! I was hoping for some help regarding shaders.

Being able to paint surfaces is the main feature of our game. We do this by giving each level 2 textures, "MainTex" and "PaintTex" which we combine using a custom shader. The MainTex is the level textures, while the PaintTex is a transparent texture that our weapons can paint onto.

The problem is that we need to keep the resolutions of PaintTex low for performance reasons, which makes the paint look noticably pixelated as you can see here:

However, we're able to dramatically reduce texture resolution AND eliminate the pixel-problem by using Unity's "Nature/Tree Soft Occlusion Leaves" shader and using it's Alpha Cutoff setting. With this shader we're able to use tiny pixel art textures and let the shader smooth it out to produce results like this:

Sadly by using this shader we lose all of the features of our custom shader such as bump-map mixing. I was hoping to add this smoothing functionality from the Tree Leaves shader to my existing shader but I don't know where to start.

Does anybody know how to accomplish this? Thanks for any help!

TL/DR: Does anyone know how to make a shader that smooths pixels like the inbuilt Leaves shader?