xrandr --brightness doesn't work in Wayland.
Minimum brightness setting is scorching my eyes right now.
SnapOverflow
Looks like there is no true solution.
Best alternatives are to use @Rober Riedl's comment and switch to gdm with /etc/gdm3/custom.conf:WaylandEnable=false, or wait and upgrade to 18.04, which will leave wayland because of its many problems.
I also installed some nighttime dimmers to somewhat reduce the brightness, but they're far from properly addressing this problem, too.