Is there really no way of inputting modded biomes? Putting in the mod namespace and then the biome id doesn't work, putting only the biome id doesn't work. It really limits the usefulness of this mod if this isn't a feature

They should work same as vanilla biomes, but I haven't tested with any modded biomes and I haven't worked on the mod in quite some time, so I'm not certain. When you say biome id, do you mean the name you see in the debug screen, like "shattered_savanah" or "plains"? That's what it needs, I believe it should always be the same as the key for the biome in the registry which is what this mod needs in order to add structures to biomes. If you haven't already, I would make a creative world with the modded biome you're interested in, tp to the biome, open up the debug overlay, and make sure the names match exactly.

If we're absolutely sure the names match, the next most likely cause is that the other mod(s) are adding their biomes to the registry after we try to add the structure(s) to the biomes. I think this is less likely than the other mod devs naming their biomes something unexpected because biomes need to be added to the registry pretty early if they want any other than ground to generate in them. It's possible I'm doing something with that registry earlier than I need to and I need to move something to a later event, but like I said, I haven't worked on this mod in a long time, so I've forgotten a lot, and it will be weeks if not months until I'll have the time to debug an issue like that

I tried with the mod namespace and without, neither worked, however I believe including the namespace should be the implementation to go for as if two mods have the same biome ID, issues will arise. Also is there support for datapack biomes as that would load last AFAIK.

Completely off topic, please mark your mod as compatible with 1.16.5 on Curseforge to make searching for it easier (very minor issue tho 😁)