Bah. A bottleneck is a component that slows down the flow of data. In this case, fps Starts with the cpu, it cannot be a bottleneck, it is whatever it is. If it can only deliver 100fps, then that's what it delivers, doesn't matter if that's with a 780 or 3090ti, it still delivers 100fps. After that it's on the gpu to either put up the full 100fps or fail and put up less.
There's 2 kinds of details. Graphic and cpu bound. Using presets like low-ultra changes both simultaneously. Things like viewing distance are cpu because the cpu is responsible for creating the frame and filling it with objects. If you have a short viewing distance, that means the frame has less objects, so less work for the cpu. The gpu doesn't care about object count or placement, it's just got to fill those pixels with color, and color changes don't matter.
So what you should do is set the in-game graphics to ultra. From there, choose a detail/slider and turn it to low. If fps really doesn't move much, put the slider back as it'll be graphical not cpu. Next slider. Keep doing that until you get a good balance, good details visually, but also good fps. Lighting affects, viewing distance, cloud detailing, grass detail are cpu, it affects objects. Character detail, bloom etc are graphical because they affect details of an object.
Use DDU, get your driver's straight, customize details and stutters should go away.