Can I turn native automatic brightness on or off from the command line?

Yes. BetterDisplay exposes the native automatic-brightness setting through its CLI for a connected display that supports it. For example:

betterdisplaycli set -name="Built-in Display" -autoBrightness=off
betterdisplaycli get -name="Built-in Display" -autoBrightness

Replace the display name with the intended display’s name. Use on instead of off to enable automatic brightness. BetterDisplay must be running with CLI access enabled. See CLI examples.

The display must support native automatic brightness. Also enable Enable native auto brightness management in Settings > Displays > [display] > Device Control > Device Control Settings…. If you prefer to leave this management disabled, change automatic brightness in macOS System Settings.

This command changes native brightness based on ambient light. External-monitor synchronization rules are separate.

Another app, system action or automation can change this setting again later. See native auto brightness management and external brightness synchronization.