Your motherboard supports the 5xxx series of CPUs, so I would look at a Ryzen 5600 as your best bang/buck upgrade. Upgrade your BIOS to most recent, then go shopping
The 5600 is better than the 9900K. That PS isn't worth 600 pounds at all. When a cheaper PC can beat it, you can't price it above said PC even if the parts originally cost more. They hold no intrinsic value other than performance, which they are outdated as. 9900K is still decent at least, but beaten by 120 pound PCUs.