I’ve decided to have the scene without a roof (Like a level in a isometric RPG) so I was unsure how to implement light portals. I tried with a large one from above but saw no improvements and even increased my render time by 50%! I disabled them now and will read up on usage.
Regardless, I updated the scene with the new LOD B building blocks. Went with a curvier design for the pieces and the scene already looks so much better than LOD A!