Let me show only specific wordpress groups (using their sub-groups nested), so for instance:

  • Primary Category 1

    • Sub Category 1.1
    • Sub Category 1.2
  • Primary Category 5

    • Sub Category 2.1
    • Sub Category 2.2

... I'm able to hardcode it within the sidebar template, but Let me determine if there's any existing wordpress plugin/widget that enables this in order to easily manage category selection/update via admin panel.

Thank you

You are able to make this happen while using wp-list-categories tag. I am presuming you are personalising your template which emerged?

The function has enough parameters to personalize it since you need.