[help] Randomizing Surface Material in Custom Biome

Hello. I am working on a custom biome and would like to randomize the surface material but am having some trouble. I am currently using a new, placeholder block as the surface material of my biome. This block is then replaced with the blocks I want to randomize the surface with in a procedure on the "update tick" trigger (with random ticking enabled on the placeholder block). The problem is that since the replacement is triggered randomly players can still see the placeholder blocks sometimes. How else would I go about randomizing the surface of my biome? The "when block added" trigger does not seem to work with naturally occurring blocks. I would also like to avoid using structures to randomize the surface since that does not guarantee that every surface block in the biome is affected. I appreciate any help y'all can offer.