ddcci-driver-linux VS ddcui

There is an out-of-tree kernel module that exposes the DDC-CI interface as a regular brightness control, so any laptop-oriented brightness control tool will work afterwards.

Those tools use the kernel backlight system, which doesn't support external monitors out of the box. It's the same with Xorg. You can get the ddcci-backlight driver: https://gitlab.com/ddcci-driver-linux/ddcci-driver-linux/

To add to this: if you install ddcci-driver-linux GNOME should be able to control your brightness again. This removes the need for any extensions, and it's what I've been using for my monitor for the last few years.

There is an out of tree kernel module doing just that (integrating ddc/ci, the control channel for external monitor*, with the backlight system). And someone is working on upstreaming it

This is kinda a self promotion but for the brightness issue https://gitlab.com/ddcci-driver-linux/ddcci-driver-linux may fix it. Or https://gitlab.com/YusufKhanAbdulHalim/ddcci-driver-linux/-/tree/master may fix it(if the first one doesnt work use the second one).

There is an unofficial Linux kernel module that allows userspace control of brightness (in a similar way to how laptop brightness is controllable):

https://gitlab.com/ddcci-driver-linux/ddcci-driver-linux

It's not the answer to your question, but you can use: Brightness Control: https://store.kde.org/p/2015475 or ddcui: https://github.com/rockowitz/ddcui for now.

This widget uses ddcutil (https://www.ddcutil.com/) under the hood. You can just use its command line interface or build a GUI wrapper around this, which is exactly what I did here. There are also GUI applications like https://github.com/rockowitz/ddcui available, though they lack DE integration of course.

There's also a GUI for ddcutil called ddcui if you happen to need that. There might be more unofficial ones out there, just search for "ddcutil GUI".

You can use https://github.com/rockowitz/ddcui instead of having to fiddle with buttons on your monitor

On Linux there are ddcui and gddccontrol (from ddccontrol), although they probably aren't as polished as the options for Windows/macOS. There is also the non-mainline ddcci driver, which lets Linux systems manage external monitor brightness just like it would internal screens on laptops.

https://github.com/rockowitz/ddcui

Would suggest DDCUI https://github.com/rockowitz/ddcui