up to 2km use the current scheme
beyond that use single textured quads always facing the camera, no effects like moving and shadows.
That should give you an idea of what perf improvement to expect and whether there is mileage in it.
Not enough I suspect.
Anyway - that's fairly similar to what my old code did, except for the 'replace four crosses by a single quad showing four trees' (getting rid of another 75% of the remaining quads...) - so I know how to do it. Should I try to dig up that code for you?
I guess it's best to let the shader decide not to render the other two.
Yes - you catch it early on and simply move it out of the view frustrum before determining light or fog.