Although I’m not so deeply involved in CesiumJS or applications that could find parts of CesiumJS useful, I have one very high level comment:
If CesiumJS is split into multiple parts, then one could at least consider to align this with GitHub - CesiumGS/cesium-native . This does not mean that the mapping between “classes” and “libraries” has to match perfectly in both environments. But I think that from the perspective of consistency, documentation, and discoverability, it could be good to at least have similar “concepts” or “domains” being covered with these libraries.
As such, most of the classes from the list above would fit into some CesiumMath- or CesiumGeometry library, but some of them could reasonably be moved into a CesiumGeospatial library. The latter would likely depend on one of both of the former, but would only be required by applications that want to do more than pure vector math.