I don’t even see the issue (off mobile, probably why).
Generally you dont use a landscape for the border skirts, because it looks empty and unfinished.
What you can do - depending on lighting mode - is to make a nice scene, screenshot it, and generate a cube map to use for rendering fake distante areas ghe player cannot reach.
Or, you can dress the scene up with high quality well pefroming distant lod billboard foliage.
Then - because the player never hits these zone, you also normally convert it to a mesh, and thus gain a bunch of performance back.
You probably want to just ditch the landscape altogether anyway, performance is better spent elsewhere…