I am having the common "missing brightness controller" problem in Ubuntu (no brightness controller in settings, or as a shortcut keys, brightness controller software does nothing,controlling brightness from terminal doesn't work)
To cut a long story short I have tried many solutions documented here, and the one shown here.
The one ray of hope is when I boot my computer with kernel 5.4.0-41 the brightness controller is there. When I use the default boot with kernel 5.4.0-42 the brightness controller is not there.
What I also notice is that when I run the command nividia-smi in the 42 kernel I get the following error message:
NVIDIA-SMI has failed because it couldn't communicate with the NVIDIA driver.
Make sure that the latest NVIDIA driver is installed and running.
In 41 it is fine and returns valid results.
I believe that last time I fixed the disappearing brightness I forced a boot with an old kernel, but this was only temporary until the next update and the brightness disappeared again. Kernel -42 is the latest version from apt but only boots with low brightness and is hard to see in normal light conditions.