How to render Bloom / Lens flare in Movie Render

I want to render the bloom/flare separately and in my experience this is usually only possible by rendering 2 passes (with/without) and using a difference matte to extract it.

When I’m lighting my scene and flip Game Mode on/off I can see a pretty different lighting look because ( I believe) of how i’m handling my auto exposure and Bloom settings in post process and camera. When I render I get the non-flare version that is slightly darker but otherwise ok.

Anyone have tips how to do so? If game-mode ON/OFF looked the same but only the flare/bloom being different I’d simply difference them.

I’ve read about Game Override, and tried changing Override to GameModeBase, GameMode, and MoviePipellineGame but no luck.

Thanks in advance