While I don't see a lot of such machines. Did you look around on any controls to push it back up a notch?
https://www.google.com/search?q=imac+screen+dims+after+booting finds this is not a new problem but many point to hardware issues.
If you decide to have it fixed, nod to Rossmann in New York.
A few minutes after startup from shutdown or from sleep mode my screen suddenly dims slightly. it also runs much slower then it has in the past.
The computer is 9 years old and running High Sierra 10.13.14