You could use swayidle and add a line to use either light or brightnessctl to dim and restore the brightness. Or just as I do turn off display after a certain time. This link should give you an idea.
wiki.archlinux.org/title/Sway#…

Are you running arch on the metal or through crostini/crouton? The second option probably will only respect chromeos' power management settings, but you may be able to flash a full uefi payload and get rid of chromeos completely with mrchromebox.tech/#fwscript

Yeah, i got all that (coreboot+arch)