Just warning you about UndergroundBiomes, but we're possibly ditching it on Resonant Rise in the next release, due to the massive amount of lag it creates when generating new chunks. If somebody using UB pregens the world, then it should be alright until they go far enough out. But if they generate by exploring... well, things go down the toilet pretty quickly.
It adds really nice blocks and a lot of variety to the underground, yes. But it also murders servers painfully when you generate chunks. And since this is supposed to be a lightweight modpack you're making, here... I can't recommend UB's inclusion.
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