I think I answered my own question after looking at the crater field premade biome. Looks like in order to have multiple blocks in a substrate you need a noise like this one:
and then you invert everything and split out the noise into two seperate noises from there so that you can fill in the black bits like so:
and the inverse:
In my experimentation I just wasn’t getting that complex but I totally get it now.
I haven’t toyed around with sub-biomes yet but I’ll try to get to that soon because that sounds awesome!
Thank you so much for all your help with my learning Luca!