I did it with this ...........
cat /etc/systemd/system/startup-brightness.service
[Unit]
Description=Set lower brightness on startup
After=systemd-backlight@backlight:amdgpu_bl0.service
[Service]
Type=oneshot
ExecStart=/bin/bash -c 'echo 25 > /sys/class/backlight/amdgpu_bl0/brightness'...