Use ffplay to check how much to brighten video before using ffmpeg?
You can use simple filtergraphs easily with ffplay.

Example using colorlevels filter in ffplay:

ffplay -vf colorlevels=rimax=0.902:gimax=0.902:bimax=0.902 input.mp4


Then you can use the same filter in ffmpeg:

ffmpeg -i input.mp4 -vf colorlevels=rimax=0.902:gimax=0.902:bimax=0.902 -c:a copy output.mp4


This example will stream copy the audio.