As you said, a unit can have a maximum sight range of 1800, there are no exceptions.
For such a big sight range a visibility modifier which is updated every second or so is certainly the best choice.
Yeah, I know that, but I want units to still be unable to see behind trees.
Or convert Coordinates to Regions and use:
And then turn it on if disabled.
Visibility - Create an initially Enabled/Disabled visibility modifier for aPlayer emitting Visibility across TheRegion