See title, I think it would be neat (definitely more accurate) if the overworld (maybe also beneath and nether) extended further downwards and also extended higher, though I don't think it work with default RTG

RTG is not the only mod (not even the hardest one to solve, even) that is incompatible with CC
actually you can use DimensionalLayers
CC is mostly no longer maintained for 1.12.2, so we would have to patch all incompatibilites for every mods in susy, and also, every mods getting added to susy in the future

I mean if someone is willing to do that I'd be more than happy to include it in the pack, but it's no the case sadly.