Hardware issue.

What hard disks do you have?

A WD blue SN570 500GB, and a WDC WD10EZEX (i'm writing the data to the EZEX)

It's a dramless SSD, I have a vague memory of seeing other dramless SSDs having some setting enabled that crippled performance on Steam. I'd do some research into that. A little tweaking may fix you up.

Note how do know your 'disk usage is low' are you measuring by the bytes written or actual wait times. If you're using bytes written, that's more or less useless as you can be 100% on disk IO but your read/writes might not be as high as you think.

99% of the time this is due to your anti-virus locking the files and slowing down the patching process which slows down the process significantly. Uninstall anything other than Windows Defender

One thing you can try is disabling write cache (in Windows) for the SSD you're installing to when downloading games through Steam. In the past some people have found this speeds up the disk write dramatically.