Be very careful if you want to disable ACPI, as even on some desktops the thermal kernel module depends on it:
Thermal Zone (ACPI_THERMAL)
This driver supports ACPI thermal zones. Most mobile and
some desktop systems support ACPI thermal zones. It is HIGHLY
recommended that this option be enabled, as your processor(s)
may be damaged without it.
To compile this driver as a module, choose M here:
the module will be called thermal.
Symbol: ACPI_THERMAL [=m]
Type : tristate
Prompt: Thermal Zone
Defined at drivers/acpi/Kconfig:231
Depends on: ACPI [=y] && ACPI_PROCESSOR [=m]
-> Power management and ACPI options
-> ACPI (Advanced Configuration and Power Interface) Support (ACPI [=y])
-> Processor (ACPI_PROCESSOR [=m]
Selects: THERMAL [=m]
So better try to find out how your system settings control the brightness and find a way to set it automatically at the level you want at start up.