Earlier quoted context omitted.
Sorry, I am not quite sure what that is. Sleep works great with my PC (motherboard is ASRock WRX80 Creator) and M1 MBP, but I can only adjust brightness through the OSD.
There’s a Mac app called Lunar that uses DDC (a protocol for sending commands to a screen) to allow you to adjust the brightness and volume of an external monitor with the usual keys on your MacBook keyboard (in clamshell mode at least) or with a slider. Very convenient, can’t recommend enough.
Personally, I rarely adjust brightness on my desktop displays and my ZMK keyboard volume buttons work reliably cross-platform.