Hey, thanks for the reply. I think this was fixed by switching the Targeted RHI to DirectX 11. Before doing that, I actually couldn’t even load the level in PIE - Separate Window mode, or the same crash would happen.
Edit: After testing more builds this was in fact caused by THE HLODs 
According to 5.1 docs, landscapes are not supported by HLOD actors. I had run World Partition > Build HLODs without changing anything, so it pulled in the massive landscape. I suppose landscape actors have to be excluded from the HLOD layers.
Said docs: World Partition - Hierarchical Level of Detail in Unreal Engine | Unreal Engine 5.1 Documentation