I know the recent iMacs do not have Target Mode. But what is the alternative for that? I would like to either use my MBP as a second monitor for my iMac or the reverse? Are there any apps for this? I want to extend the screen not mirror it.
To add to this, I know Elgato is a good brand. But OP is going to be limited to 1080p 60Hz using their USB options. No idea if it's possible to use their desktop cards in some kind of an eGPU enclosure or something, because those are the only ones they make that can currently do 4K 60Hz. Probably won't find a 5K one. Maybe if they moved to USB-C 3.1 (10Gbps) they could do higher but IDK if that would be enough, and I'm not sure if OP's iMac supports that. They would have to release a new USB capture box for that option to even exist. Anyway, to get the live feedback, OP would need the HD60S which again is limited to 1080p 60Hz. Better than nothing I guess…might be worth a Google to search for a Thunderbolt option that would have the pass-through.Yeah, there's an option. Get a capture card. Plug the output from the other device into the card, plug the card into the iMac.
You'll want to be sure that you can make the preview of the capture full screen, and that there's very low latency. I can't speak to either one of those with specific model products.
Brands to check are BlackMagic, Elgato, Magewell.
These devices are mostly made for live streaming. So even if the software it comes with doesn't go full screen, another software might.
I'd start on Amazon, search "capture card". Good luck, let us know how it goes!
Is there currently a device the OP could plug into his external display port to make the Mac think and behave as if there's a second monitor?
If so, it would probably be possible to use screen sharing on the other Mac to display the "phantom" screen.
To add to this, I know Elgato is a good brand. But OP is going to be limited to 1080p 60Hz using their USB options. No idea if it's possible to use their desktop cards in some kind of an eGPU enclosure or something, because those are the only ones they make that can currently do 4K 60Hz. Probably won't find a 5K one. Maybe if they moved to USB-C 3.1 (10Gbps) they could do higher but IDK if that would be enough, and I'm not sure if OP's iMac supports that. They would have to release a new USB capture box for that option to even exist. Anyway, to get the live feedback, OP would need the HD60S which again is limited to 1080p 60Hz. Better than nothing I guess…might be worth a Google to search for a Thunderbolt option that would have the pass-through.
Yeah at that point might as well buy a new display. Even if you can get all of that up and running you’re still dealing with an older display that has a limited lifespan and will probably go all weird on color shift. At least that’s what my old displays tend to do.Ah, true. I wasn't thinking about the resolution. OP would need a 4K capture card from BlackMagic. And then a PCIe to Thunderbolt expansion chassis. And that all starts getting a little pricey.