Trying to create Superflat world with grass and trees

What I want to create is basically a superflat world made out of 1 layer bedrock and 20-40 layers of some white block and on the surface there is custom grass, sparsely custom trees and occasionaly rocks. However I cant seem to generate structures in a superflat world. How can I generate custom structures in a superflat world or is there an alternative with a biome that spans across the entire world which is relatively flat and that does not generate ores caves, water or anything except my custom structures/blocks?

I tried to set the white block to sustain plants and made the grass "plains" type instead of previously "cave" still doesnt generate in a superflat. However in a normal world/in a buffed world with this biome it generates. I tried to set the superflat preset with ;decoration and its not generating when I set the white block "can plants sustain" it generates vanilla grass and flowers.