It should be easy enough to get the power consumption down to less than 8 watts at idle with the screen at minimum brightness if you are using power profiles daemon for power management. Screen brightness does have a non-linear effect on power consumption though. If you use brightnessctl to set the underlying value between 0 and 255, a value of 50-80 is perceptually “50%” brightness, but consumes about a quarter the power of full brightness.
There is also the ABM feature for the panel that adjusts brightness based on screen content, and it is very subtle at lower levels (I have it at 1 out of 5), and that can save some battery too. See Mario Limoncello’s excellent work on that over on this thread: Adaptive Backlight Management (ABM)
TL;DR it should be coming to mainline linux in 6.9, or maybe later, and it should already be supported in PPD 0.20.