I'm using a custom data pack to create a void world and even though I changed the base biome type of my worlds single biome to "ocean" I haven't seen a single ethereal island spawn no matter how far I travel, and this is with me setting the spacing to only 5 chunks.

My suggested fix for this would be to either provide a setting where we can choose which biome(s) the ethereal islands spawn in, or one that allows them to spawn in all Biomes.