Hi,
two are not related, the "Cumulative update cache" component has its description, it should be needed only when switching Windows branches, to reapply the latest CU, and not need to redownload it.
If anyone has better information, I'm all eyes.
As for 1903 reserved space ("storage" actually), you can only disable it before install at this point, there is a separate Setting/tweak for that "Reserved storage".