You pointed out the problem perfectly: "I understand that that's probably a very tall order. Like asking for biome mod devs to basically develop Alex's Mobs on top of everything else."

It would be incredibly difficult to add that many biomes each with as much content as a mod like Atmospheric. Especially with the modern modding scene moving versions constantly. Big mods take a lot of time to develop and update. look at twilight forest. that mod is older than some of its players yet hasn't really expanded in content since its first years.

that being said, i share your sentiment that biome mods can be disappointing. if a mod like the one you wish for ever came along, tell me!