There are workarounds, like "Luna Display" (I haven't tried it and I'm not affiliated). I also wonder how well running something like a VNC client fullscreen o

There are workarounds, like "Luna Display" (I haven't tried it and I'm not affiliated). I also wonder how well running something like a VNC client fullscreen on the old iMac with universal control might work. (I've also used Jump Desktop's "Fluid" protocol which is the same general idea as VNC, though provided a higher-quality lower lag connection in my case.) I think there are some decent solutions for a secondary display, but I kind of doubt any of these would be good enough for a primary display for most use cases. I would guess all of these have tradeoffs in terms of lag, frame rate, quality, reliability, etc. though I'd love to hear different.