A trick i came up with is to decrease the distance to the horizon (so you can use a smaller EcoSystem terrain, plane or set of planes) by tilting the far edge of your terrain or plane upwards. You can tilt quite a bit before it becomes noticeable.

You can also use a line of hills or mountains (small scale and fairly close to the camera) to cover up the horizon. Spectral lighting or other haze and fog can make the small scale terrain look far away.

I usually use a set of planes that barely cover the camera view, with the level of detail of the objects in the farthest planes set the lowest.

When you need terrain, procedural terrains have an automatic LOD (Level of Detail with distance) feature, so use those instead of true meshes.

Sometimes 2 simpler terrains poking through each other can appear more complex than a single complex terrain, even though each is much less than 1/2 as complex - a synergistic effect.

Decimating objects before adding them to an EcoSystem might help.

The above is written from Vue Infinite experience and might not all apply to other versions.