Hiding background of a video with fire

Hello everyone, I'm working on a video of images that catches fire. (selling images) I want to opacify the background because it is lighted as it goes through combustion.

I'd like to keep a realistic side with the work of light coming from the flames.

The idea of having a neutral black background.

Not seeing too much how to do it, I work image by image with the painting tool in order to mask as and when it goes along and by playing on the opacity but the "actions" of painting on the layer make the video heavier and I get back again with a rendering every time in a long time.

Do you have any ideas? I'm a little desperate to finish this video...

I would point out that I am starting with after effect, thank you all for your advice.