What iMac do you have?
Depending on the year and OS, there are some solutions involving the Apple Thunderbolt Monitor. No one knows if they still work.
Otherwise, there’s an App called EasyOnTheEyes in the App Store. It’s free and seems to work across the board.
There is no 3rd party monitor I know that supports control from Displays. I could be wrong but I can’t find anything from anyone.