So first, I think that is a great homebrew solution. I myself have absolutely no issue with GWM but feel SS gives too much for a single feat. I have never had enough of an issue with sharpshooter to bother changing it but I have always felt it would have been better split into 2 separate feats. I think your solution is even better.
So as far as this, I see this argument a lot. There is a very simple solution to this issue. First, don't build encounters on an endless, flat, featureless plane. Put your bad guys in the woods, in a camp, in an outpost, in an area with shubbery and/or a natural trench in the terrain. Second, any intelligent creature is going to be well smart enough to not stand there taking arrows in the face. Even the less intelligent creatures living in the world, if they have lived long enough for your characters to encounter them, must have at least some survival instinct. Heck, my one dog used to make a loud fart and run and hide not smart enough to realize it was him that made the noise and even he as smart enough to run and hide on the porch as soon as I would fire the first bolt from my crossbow when target practicing. And lastly, I usually add 1 extra monster to my outdoor encounters as I know one is going to die immediately, if not from ranged attacks, then because my party snuck up and inst-gibbed one in the surprise round.
If my players start firing from 600ft or decided to fly up 600ft and rain down arrows, once the first monster was hit they would have nothing left to shoot at. The monsters would dive behind/under cover, whether that is a rock wall lining the area, a cart, a tree, a building, or whatever else was appropriate for the setting, even one of them with the ability to cast fog cloud works. Never even once have my characters been able to stand back and kill an encounter from 600ft.