I don't know the answer offhand, but one thing I would do is see whether Mpv is equally effected. By default, the number keys 1-8 will control those settings.
You can set the default brightness in mpv in ~/.config/mpv/mpv.conf like this:
brightness=-10
Look in the manpage (man mpv) for other settings. I think any option like --brightness=-10 can also be put into mpv.conf by removing the -- at the beginning.
I don't know if there's a way to make mpv autosave this.