Happy this helped 
Maybe you could try to reduce the emissive from the fire ?
Do you have lights in your particles ? (Not a pro in Niagara)
I can see in your screenshot that, those options are in the project settings, have you tried to adjust them in the post process volume ?
I don’t see a lot of reflective material in your scene, have you tried to use Screen Space reflection to see if the issue persist ?
Also, try turning off ‘use hardware Ray Tracing when available’ to get a true feel of Lumen. This option give you Raytracing even if you try to use Lumen - which might give you different result (for instance, on play mode your image might be brighten up (at least it does for me).
Hope it help!