Low CPU usage during rendering

I can't tell you exactly what's been happening. How much is your GPU utilized during those same tasks? If it is much higher than your CPU utilization, there is a good chance that you have a performance mismatch between the CPU and the GPU. But if that GPU utilization is also low, then you likely have something else bottlenecking your CPU.

And all too often I see people configuring a non-gaming, video editing system go heavy on the CPU but cheap out big time on the GPU. This actually degrades everyday app perfoprmance, not just video editing performance: In this scenario, the CPU usage will be capped far below 100% no matter what.

And that RX 590 dates from a time when the most powerful mainstream desktop (as opposed to server and workstation) PC platforms had only 6 to 8 cores in their CPUs. The 12-core Ryzen 9 3900X was not released until long after the RX 590 was superceded by newer GPU architectures.