Connected an external screen, it was dark, googled for a solution after resetting redshift settings didn’t work.

So, there are a lot of ways to change brightness (SO¹).

xbacklight works with hardware-level brightness for the devices that support it.

For the others, software-level changing of gamma values is what’s usually needed, and what I did with a lot of different programs before. This worked this time:

xrandr --output LVDS1 --brightness 0.5

(As a bonus, it uses the already well-know and well-loved xrandr.)

Sad that arandr can’t do brightness though, but there are reasons (missing –brightness features (#35) · Issues · arandr / ARandR · GitLab)

From there I learned that ddcondrol is the way to change brightness for external monitors on hardware level, and that Jolmberg/wmbright: Monitor brightness control dockapp is a back-end that tries to do everything.