There's currently a command to do this, so making the brightness slider change the brightness of SDR content might be a good idea (as most apps/games will be SDR, so adjusting the brightness of them is useful).
A possibly relevant merge request was started @ https://invent.kde.org/plasma/powerdevil/-/merge_requests/356
Git commit 9d76ee3831a4b05580e2928e396f3c74e61d6d5a by Xaver Hugl. Committed on 14/05/2024 at 08:10. Pushed by zamundaaa into branch 'master'. daemon/controller: add support for KWin brightness control This works with HDR for now, and can be extended to take care of other methods of brightness control later on as well M +1 -0 daemon/CMakeLists.txt A +130 -0 daemon/controllers/kwinbrightness.cpp [License: LGPL(v2.0+)] A +57 -0 daemon/controllers/kwinbrightness.h [License: LGPL(v2.0+)] M +2 -0 daemon/controllers/screenbrightnesscontroller.cpp https://invent.kde.org/plasma/powerdevil/-/commit/9d76ee3831a4b05580e2928e396f3c74e61d6d5a