One thing that would help quite a bit on Windows would be allowing the player to select the screen the game is on

Can be done with the following APIs:

DisplayServer.WindowSetCurrentScreen();
        DisplayServer.WindowGetCurrentScreen();
        DisplayServer.GetScreenCount()

I think the default should be kept as automatic window placement (this could be -1 when saved in the settings) and >= 0 values would then once apply the window position so that the game still allows manual changing of screens. Though also if the setting value is changed then the window should be moved.