I’m embarrassed that I don’t know the answer for this, but here goes…
I currently have an iMac and extend the desktop to my iPad. I’m about to get a new laptop for work and wonder if I can I do that in reverse? Can I have the laptop extend to the iMac, effectively using it as an additional display?
How I miss target display mode. I can recommend the Luna Display. I use it everyday for work. The only downside is I have to jump through a few hoops by extending my desktop from my work MacBook Pro to a Studio Display and my iMac by making sure I do the Luna Display first, then connecting the Studio Display.
Ventura “Screen Mirroring” also known as Sidecar should work.
But even better, IMHO, is to keep running the iMac and use Universal Control so that you only need one mouse and keyboard to work with the laptop and the iMac. I’ve been doing this for the past few weeks and am loving it. My iMac serves as my “status board” while I do all my work on my laptop and its external monitor.