See Luoshuang’s GPULightmass Settings
Never used UE 4.27, used 4.26 for reference, NO INTEREST in 4.*, to many bad experiences. All your questions on this were answered in the UE4 thread
There is much more in the original post just you have >2000 to look through!
The post about adjusting settings is
Luoshuang’s GPULightmass - Setting for baking