The RX 570 is a 'medium quality at 1080P' card for most newer games...; if you are happy that that level of detail, then keep it as long as you are content with it.... (The RX 580 is a max detail/1080P card, and even adequate at 1440P /medium in many games...)
As the RX570 is a 4 GB of RAM card, but, realistically only fast enough for medium detail 1080P, you might rarely see higher usage no matter what CPU is utilized, so, I'd not worry much about levels of usage. If your frame rates are acceptable,
that's what matters.)
Owners of many 4c/4t CPUs have been complaining of minimum FPS issues in many of the newer more demanding games, but, this will vary according to refresh rate needed (60 Hz vs. 144 HZ, etc), and, general strength of GPU.
If you could find a killer deal on a 4c/8t CPU (under $80-$100) I'd consider it...
Otherwise, I'd just plan your R5-3600/B450 board/DD4 combo upgrade like half the gaming planet is...