I want to upgrade my current PC because during a game, I start off with 90-100 fps on fastest settings but mid game to late, I’m down to 40-80 FPS. I’m currently using a Ryzen 3 2200g with no GPU and 16GB of RAM. I thought of buying a RX 570 / RX 580 but watching benchmark videos on youtube, I don’t really see any difference in FPS. Should I just buy a newer processor or should I go through with buying a GPU?

Note: I cannot afford to buy a whole different PC. I could only afford to replace parts one by one. Thank you for reading and thank you in advance for answering.