/Settings/Gui/Brightness is being set to a max of 100 when Venus OS boots.
The maximum value of the Raspberry PI 7" touchscreen is 255 so the maximum brightness available is about 40%.
You can set the brightness to a higher value in the Display menu but it is set back to 100 after boot. dbus-spy will only allow values up to 100 as well.
Restarting the gui (killall gui) resets the max to 100.