Dear valued Unreal Engine user,
Yes, you are correct in that -nullrhi will disable the renderer.
If you wish to use offscreen rendering, then this will depend on the platform and RHI chosen.
If you are using Vulkan on Linux or D3D12 on Windows then it looks like you can use the -RenderOffscreen flag. At a quick glance through the code, it looks as though this may not yet be supported on Metal on macOS, so something you may need to add in yourself if you need to use that platform.
Thank you for your continued support,