Ugh… Biome is the most tricky thing to mod right now I think. Some weeks ago I was trying to mod a terrain, but somehow my trees ended up not appearing, or only some of them appeared even though the parameters looked fine. I don’t know if it’s due to the collision boxes or what, I was just trying to get loooots of trees and the engine decided to do the opposite.
I think what you describe could be a thing of the seed. I don’t know if there’s a way to make that volcano-like structure you mention on every single seed. 
@BrunoSupremo (from the archipelago biome mod) might know better.