Ok, here is what I did: I created a mobile skylight with a custom cubemap as light source (the scene capture did not work well because it had too much color from the nebulas in the galaxy). Then I changed my wall lights to point light with no more shadow and made sure that at most two lights can overlap with each other.
I also used Nawrot’s advice to create an unlit material for the planet - now the planet uses a layered material (daytime and nighttime) that is blended depending on the sun actor’s position in the game. If anybody is interested in it, then I can upload the material network.
This is the result, with a solid 120 FPS 
Thank you!
