We could include an switch in the Brightness applet to control the brightness of all screens simultaneously.
Interesting comment by rockowitz (ddcutil author): https://github.com/rockowitz/ddcutil/issues/336#issuecomment-1732603986 which can be taken into consideration:
I believe what you're asking for is to have ddcutil simultaneously apply the same changes to the brightness, contrast, color etc on multiple monitors. This unfortunately is not possible.
First, ddcutil cannot control the laptop monitor. eDP devices do not support DDC/CI.
More generally, ddcutil does not know how to modify settings on different monitors to achieve the same visual effect. For example, the maximum brightness value differs between monitors. Even if the values were normalized, e.g.. as a percent of maximum value, using the same normalized value would not set the same absolute brightness on all monitors. Furthermore, brightness changes are not linear in their effect. For example, suppose that a normalized value of 20% on monitor A produces the same brightness as a normalized value of 30% on monitor B. There is no guarantee that normalized values of 40% on A and 60% on B produce the same brightness.