Custom Materials Based on Screen Position

My goal is to render a scene such that some portion of it uses high quality textures, while the rest of the screen renders using lower quality textures, either mipmapped or manually generated ones that are the average colour of each texture. For instance, I would like to render the center of a scene like this:

While rendering the outlying edge pixels with basically flat textures; here I mipmapped all the texture maps within this particular material:

I generated those pixels using a Material Collection, but the issue is accessing when to change the material value. Doing it via a level blueprint with Read Rendertarget Get Pixel would probably be too slow for my use case (the comment says it’s ridiculously slow), but I can’t access specific pixel positions in Material Shaders, obviously. Any help would be appreciated.