Good work on the backdrop island find, i have more faith in the distance cull setting now, i will do the same thing on mine. Maybe they are so far away they aren’t getting added to the HLOD ???
With the ocean, check how many waterbrushes you have in the outliner, there should be only 1 per landscape.
Keep playing with the ocean splines, there could be a sweetspot.
I notice you also have an underground volume in the Red BOX, I wonder if this has any influence on the situation