This is more of a quick helper for anyone else who had similar issues regarding Blender rendering the entire scene instead of just the render border that you've selected.
Make sure that within your "Render Properties (camera icon) > Dimensions Panel" that you have "border" checked.
Go into Camera Perspective ( 0 ) in your 3D viewport, ctrl + b to go into render border selection mode, and select your render border.
My problem dealt with me attempting to draw the render border in User Perspective mode, but the render border select tool seems to only work whenever I am selecting a border in Camera Perspective mode.
