Hi Yourself.
Don’t use cube maps / HDRI. It is a total hack approach.
Use a directional light for the sun, and a simple skylight for general sky light. Balance the two then search these forums for the best lightmass settings. BNy default they aren’t ready for production results.
I’ve looked into using Unreal for advanced lighting, and it isn’t quite ready yet. HDR’s in skylights don’t work… they make a general colour approximation, but make no directional shadows.
Lightmass also tends to ‘dumb down’ the GI product. Everything is smoothed out ALOT, and rays don’t bounce ‘correctly’ as they would in tracing engines, i.e. arnold/redshift Maya. This is a good thing for getting started… a bad thing for people that want realism.
good luck!