Hello, 84rt-design,
I am going to send you a link to the Epic Wiki. This particular section has an extensive library on lighting and troubleshooting. It also includes areas that emphasize lighting and optimization.
I can tell you from experience that grass is really thick. You may consider thinning it out some.
If you look at any area of grass you will see breaks where there is either dead grass or patches where the grass didn’t grow and it looks bare.
This is also a good way to break up the grass itself allowing for the scene to breathe and appear more natural.
Thanks,