And when you say the serial number is not unique, even if it's included, do you mean that's because the display can be connected through multiple connectors at the same time?
Yes, that is a possibility, but the more practical issue is that noone enforces the serial number to be unique. Some manufacturers have display models with all the same EDID.
In the long run, it would be nice to have access to a unique identifier
We could generate one per output and store it in KWin's output settings file, but on the Wayland side we don't need a unique identifier anyways, as the brightness value is stored in that same output setting file already.