Do I need a Dolby Atmos renderer or can I just use Immerse?

No, you need Apple's renderer. 

You will need a Dolby Atoms renderer, and the example of Dolby Atmos renderers are:

Dolby Atmos Production Suite

Dolby Atmos Mastering Suite

Nuendo / Cubase / Logic 

These are the options for the Dolby Atmos renderer, and all of these are compatible with Immerse workflow: 

Option 1: The Mastering Suite does have Windows compatibility, so it would be the primary alternative for a full Windows + Pro Tools Atmos production workflow.

Option 2: Nuendo and Cubase offer a native Dolby Atmos Renderer and great Windows compatibility.   Both have free 30-day trials and are fully compatible with Immerse Virtual Studio for your Atmos monitoring.

Option 3: We have a hunch there may be a workflow for mixing simple Atmos Beds in 7.1.2 and rendering that directly in Pro Tools to an ADM file for delivery.  The trick would be to assign only Beds (i.e., 7.1.2 mix).  Our team is looking into whether it's possible to render an ADM file containing only Beds based upon this workflow.