My pc has a Ryzen 7 2700x with a GTX 2070 super, but whenever I go into a PvE server with a lot of lights, my fps goes from 70 on average, to around 15. But here's the issue, my GPU usage is only at 40 percent on average. So it's not even using that. How would I get better FPS at places like this? Could I get rust to use more of my CPU and GPU?

You might be able to set rust's priority in task manager, you can just google for tutorials for how to do that. I'm not sure how much of a difference it will make though.

What resolution are you running at? What does the task manager performance tab look like?