It is a commonly-repeated fallacy that there can be no CPU-related bottlenecks at 1440p. This is a misunderstanding of how the rendering pipeline works.
1440p results in lower CPU-related bottlenecks for most hardware combinations because with everything else being equal (the same game with the same quality settings) gaming at 1440p will result in a lower number of frames per second due to the higher load on the GPU that the extra resolution places. That's the only reason; the load on the GPU goes up, and the load on the CPU goes down, removing the bottleneck. With your system though, your GPU is so powerful that switching from 1080p to 1440p doesn't place enough load on the GPU to keep your CPU from bottlecking you. The faster that GPUs get, the faster CPUs need to be to pre-render the frames being requested by the GPU.
The issue with all Ryzen 3000 series CPUs is their low boost clocks, their relatively low IPC (instuctions per clock/cycle), their higher internal latencies, and their overall weak single core performance. These are basically all the things that the Ryzen 5000 series set out to fix and fixing them has catapulted AMD to the top of most CPU benchmark sheets. This is why they are significantly better for gaming.
Ryzen 3000 series CPUs were always popular due to their value (more cores and threads per $ than Intel), upgradeability (the AM4 socket has stuck around for approximately 5 years) and good performance in productivity applications. They have always played second fiddle to Intel's CPUs when it came to gaming performance however. Moving up the product stack only helps a little bit; there isn't a single Ryzen 3000 series processor that can approach the gaming performance of the Ryzen 5600x.
The issue with upgrading for you is that you're on a 400 series motherboard; the Ryzen 5000 series requires a 500 series motherboard. You could upgrade to a B550 or X570 motherboard as well as to a Ryzen 5000 series CPU, but both AMD and Intel have stated their intention to switch to a new chipset and motherboard socket for their next CPUs (LGA 1700 for Intel and AM5 for AMD) so no matter what you upgrade your CPU to right now, if you ever want to upgrade to a newer generation you'll need a new motherboard (and DDR5 RAM; both Intel and AMD plan to switch to DDR5 RAM for their next generations as well). Thus, keep in mind that whatever you upgrade too now will need to be completely redone again for your next upgrade regardless of whether you go with AMD or Intel.