Bottlenecks are a very misunderstood topic in computer hardware.
The bottleneck you are refering to does not exist: you can not have a CPU that is too powerful for the GPU you are running.
The CPU has to pre-render frames before they are sent to the GPU to be rendered and displayed on your screen; if your CPU pre-renders frames slower than your GPU can render them, then you have a CPU bottleneck. CPU bottlenecks are bad not just because you don't get the most out of your GPU, but because they cause stutter and frame drops.
However, there is no disadvantage to having a CPU that is more powerful than the GPU, its prefered even, because when you have a CPU that isn't running at 100% and a GPU that is running at 100%, you get stable, stutter-free gameplay and get as much performance out of your GPU as you can.
If you want to upgrade your CPU without changing your motherboard, an i5- 8600k should be a good improvement. An i5-9400f would be cheaper and uses less power, but it doesn't have as high of clock speeds as the i5-8600k which could cause you to need to upgrade sooner down the road than you would have to with the i5-8600k.
If you're looking for a "balanced system all round," I'd recommend that you upgrade to 16 GB of RAM as well; 8 GB will limit you in games like Warzone.