What Pc parts should I upgrade?

I built my first PC in February for around 700 dollars. My goal was to get the most performance for the least amount of money, and I have done that for the most part.

The only game I play is iRacing, and I've been having performance issues in longer multi-classraces. iRacing usually runs at a stable 75 fps, but last night, about a half hour into the race, my frames just started dropping, and by the end, I was at 30 fps.

I have a single monitor at 1080p, and I run Racelab along with iRacing. I don't care about how good the game looks as long as I have a stable frame rate, and I think I need to upgrade my PC. Besides RAM, what should be my first upgrade? I know it's usually the GPU, but I am unsure since sim racing is very CPU reliant.

This is my current build https://pcpartpicker.com/list/gHWn8Q