Does it have option to add or remove certain percentage with one command?

I believe the value used when running brightness [value] is a percentage of your monitors total brightness. e.g. If one monitor can reach 400/nits and another 900/nits, then brightness 50 would set it to 200/nits and 450/nits respectively.

XF86KeySum on laptops

DDCUtil which is what this is based on unfortunately doesn’t support laptops.

is there finally desktop monitors with brightness support?

The program effectively changes the brightness via the monitors on screen display.