You can lower the spawn chance in the config (SC). If that doesn't help, you can set a spawn override (SOV) and remove some biomes from it's spawn biomes. Alternatively, you can disable Overworld boss spawns in the config (mob category) if you don't want them to spawn in at all in the Overworld.
If I disable them from spawning in the Overworld, will they spawn in dimensions added by Tragic MC? Note I haven't visited those dimensions yet so I have no idea what they are like.