I tried to fix this while using ubuntu 20.10, no success. Then, tried to boot a pre-release image of 21.04. For my suprised, it WORKED out of the box!
I upgraded my system and... no control over brightness again. I tried to change drivers, and finally tried to boot the same image again and for some reason it wasn't able to change brightness!

Currently, I am able to change brightness using an app called "brightness-controller", but it won't set the brightness permanently, it's restored at rebooting and many programs or system events max it out again. If only I could set a lower level of brightness as default, it would be enough for me.

Notes: I already tried running Pop OS 20.10, same issue.