1: GPULM quality (Preview/…/Extereme) changes only the NumPrimary and NumSecondary config values. In Extreme to the insane values.
So better just put manually 128/64 and it will be fine for most cases
2: You always should use PRODUCTION quality drop-down selection in UE4. Otherwise some direct light ( direct rays from any source) will give you bad shading at bending/curved areas.
3: beware of putting lights at very reflective elements or creating very small light sources from emmissive meshes - you will get then many white splotches on the walls
4: I have used many many HDRI and never got the fireflies. Consider to use better HDRI or check your SkyLight configuration (esp. the Intensity value ).
5: Most of World Settings are no longer utilized for GPULM. Only one parameter GPULM is using for the baking: Indirect Lighting Bounces and of course parameters for VLM. Rest is ignored.