That's a helluva deal and clever use of an old iMac. I have seen some people take the iMac LCD panel and add a 3rd party controller to make it work as if it was just a regular display.
The Apple displays are so damn overpriced for what they are and the main reason to use them is because MacOS is completely crap at scaled rendering on 4K displays.
That's exactly what I'm doing. Gut it, put in the controller/video board, run the power in through the Memory door and reseal the glass to the frame. I'm essentially making an Apple Cinema Display out of the older iMac. I also have the VESA mounts required for both iMacs.
Once the 2015 is turned into a display and tested, I'll backup the newer one, mostly gut it to install the VESA mount, then upgrade the NVME drive, add a 4GB internal SSD for backups and run, instruments and samples off of an external TB3 enclosure with a 2TB NVME drive.
Then I'll buy the new desk, install the VESA ARM kit for the desk, and then get the NI S61 keyboard controller and a few other things so I can finally get the song ideas I have out of my head recorded to more than a bunch of ideas & scratch track snippets dumped on a hard drive.
Not sure if I'll undermount the S61 Keyboard under the desk on a sliding tray or not.
NOTE: For using an older iMac in Target display mode with a 2019 or Silicon based Mac one would have to go to an older than 2012 iMac, then force patch it to at least Monterey to bring the OS up to snuff to do Target Display Mode, with the newer Apple system running Moneterey or Ventura to see it.
AirDisplay has too much lag/latency even when connected via TB2 to TB3 Adapter and Cable to make it useful from a recording perspective.
Having the mixer open on the other screen looks great but the meters and mouse pointer having occasional lag is not useful.