With regards to grasses, trees, etc, use the in-engine tools. Particularly grass-layers if you are using a landscape. They can save you time, most importantly, but also lay out low-cost meshes (no collision) vs dragging and dropping actual grass-meshes into the scene.
Things that don’t need collision, like trees in the background, likely a place you will never get to play in, don’t give them any. Keep anything you can as low-cost as you can; if the thing doesn’t need it, be sure to turn it off. This will help save you CPU cycles; not spending time checking/evaluating things you don’t need them too.
Your materials look good. I can see the units/trees are green, good. Landscape is mixed brown and green, but that’s what you get for that tool; is what it is. River is hot-pink, but that’s expected for anything near good-water, transparency or the like.