Hello,
if you really want to do parallel processing, that I suggest you do it with powershell core. That functionality is native there. And you can get a nice progress bar.
Windows Powershell Workflows are actually windows workflow foundation in the background. It works, but sometimes you can see some "interesting" side effects.
Anyway it's also doable with Windows PS with 3 -4 lines of code. :)
Let me know if you would like me to help you.