If it’s not already installed, try brightnessctl. It is one of the easiest utilities available for this feature.
sudo pacman -S brightnessctl
This and plenty more solutions are documented here: Backlight - ArchWiki
Check out ARandR for a simple GUI display settings app.
i3 is sort of philosophically the opposite of what you are describing; GUI options are intentionally avoided in favor of allowing everything to be controlled in config files or command-line applications. Still, with some digging I’m sure you can get some good GUI apps set up for controlling your settings.
Hint: a lot of XFCE apps are likely to work, without pulling in half a distro worth of dependencies like you may find with Gnome or KDE apps.