Help to change my CPU

I upgraded my graphics card a year ago to an RX 9060 XT 16GB, but I've noticed that in many games, the graphics card can be bottlenecked by the CPU—especially in games that demand both CPU and GPU.

I have a Ryzen 5 5600 processor, and I’d like to stay with AM4 because I can’t afford to switch to AM5 due to the exorbitant prices of DDR5 RAM.

Ideally, I’d like to spend no more than €300 on a new or used.

What would you recommend in that case? Thanks for any answers and constructive advice, of course!