Brightness was a mess, so hopefully the new code will ultimately simplify brightness control.
While waiting for a proper fix, you could use a script based on ddcutil. To see if ddutil works for your hardware (mine is an old iMac):
% doas ddcutil capabilities | grep "Feature: 10"
Feature: 10 (Brightness)
% doas ddcutil getvcp 10
VCP code 0x10 (Brightness ): current value = 9408, max value = 65535
% doas ddcutil setvcp 10 20000