Depending on the size and complexity, it may not be necessary as a fix for your issue.
try the Build Landscape and HLODs first, see if it works, unless you already did that just before launching memory calc.
As for HLOD, I am fixing up an old map with 7000+ actors, I just removed HLOD from all props and then did an editor fly around of each bit, selecting stuff that needed to be seen from a distance and re enabling HLOD on those.
Sounds a big job at 7000 actors, but not too bad. Only took a couple of hours. But worth it
Next map you’ll be able to set them when you place them.