Your workflow is completely wrong, there’s many things you need to change t get it working correctly
Each object gets a single lightmap, so if it’s all one object it can’t possibly get enough detail in the lightmap even at the highest resolution, you need to split up the mesh into many separate parts.
It has to load everything at once since it’s all part of one thing, this means it can’t unload parts that aren’t visible
You have objects that are reused more than once, if you import one by itself you can place the other copies in your UE4 level and that will save memory because it will only keep one in memory.