The genius part about this is your two "monitors" look the same. This is literally a significant part of why I've never purchased an iMac. Now you've given me an idea...
Target Display kind of sucked in that you needed a fully functional installation of MacOS running the entire time too. It’s nice that the feature was there of course, but I never found it pleasant to use.
The problem is the Cinema Display doesn’t look identical to the iMac. I like have monitor symmetry, but there was (maybe until the M1’s mini) a gulf in hardware between the iMacs and Minis. Especially the pros.
Frustratingly this was a native iMac feature "Target Display Mode" in 2009-2014 but they dropped it from the 5K model: https://applerepairstation.co.uk/Technical%20Desk/use-your-i...