I’m not quite sure what would cause the Engine to preload all the sublevels, so probably the first step is to work out why that is happening. Can you reproduce the problem in the Cesium for Unreal Samples project?
Also, is this in the Editor? In Play-in-Editor? In a shipping game?
Additionally, I saw in this old thread from 2022 that cesium sublevels are not really supported? Is that information still correct? Sublevels in UE5 broken
No, the sublevel system has since been rewritten for UE5. They should work well now.