How do you stack mask textures on a single diffuse?

Hi! I want to implement this effect in Unity through a Shader Graph:

What happens is that there is multiple round-ish black & white textures that represents areas of burning ground. All of them are combined through Math.Max meaning if multiple are overlapping, it is clamped to 01 and they are merged into a single bigger blob mask.

What I have tried is to feed a Texture2D Array to a Shader Graph with the intention of feeding the positions as offsets but the problem is that I can't iterate over an Array + there's no such thing as an array of Vector3.

The second idea is to merge them via code. I make a huge mask texture which I fill with black, then iterate over each mini-blob and MAX the mask pixel with the miniblob pixels. Then I setpixels, apply, and should work. Right? Wrong. It's such an overhead that my game runs at 1 fps

I must admit shaders are not my forte but I really want to make this work somehow.
Does anybody have a fix for my attempts or perhaps a new approach?