I saw in 1.21.1 NeoForge that your mod often uses the minecraft biome tags for the structure to spawn in. But haven't seen any c tag uses. Both NeoForge and Fabric provide c biome tags that you can utilize to spawn your structures in even more modded biomes! Like Giant Mushroom structure could then spawn in c:is_plains tag which has vanilla and modded plains biomes (excluding Snowy Plains which is in c:is_snowy_plains)

c namespace biome tags:
https://github.com/neoforged/NeoForge/tree/1.21.1/src/generated/resources/data/c/tags/worldgen/biome