I have a ten year old iMac at the moment that would make an amazing second monitor... but it doesn't quite have the features I need to use it like that. An HDMI input would be great.
There is a reason that is difficult to do: the licensing terms for HDMI require manufacturers to put DRM in place to make it impossible (or at least not straightforward) to record from an HDMI source. It's nearly impossible to meet that requirement on a computer.
Edit: nevermind, I see you addressed this in another comment.
In the specific case of the iMac:
- USB-C video outputs are all either DisplayPort[0]. HDMI Forum was promoting an HDMI USB-C altmode but it was never implemented. USB-C HDMI adapters are all DisplayPort adapters that additionally tunnel DVI traffic (because HDMI 1.x is actually single-link DVI with a funny connector and extra bits).
- Apple used to support Target Display Mode for video input until the Retina iMac a decade ago. They dropped Target Display Mode because their 5K display panels were using eight lanes of DisplayPort, a configuration that doesn't exist[1] outside of Apple's custom display controllers. Every other 5K display (and even some 4K ones) were instead using multiple cables, with some extra (poorly-supported) metadata to tell the graphics card to treat both halves of the display as one.
Why Apple hasn't brought TDM back now that we have enough bandwidth for 4.5K over standard cables is beyond me.
[0] Or MyDP, aka "the funny Nintendo Switch altmode"
[1] Think of it like a "32-lane PCIe port". It's technically possible but no host or device silicon supports a link that wide.
https://www.makeuseof.com/hdcp-vs-hdmi-whats-the-difference/
Not tunnel, convert: these are active adapters that take in a DisplayPort signal from the computer and re-encode it as a DVI/HDMI output signal. The computer doesn't need to know or care that there's HDMI happening downstream, because it's still just outputting DisplayPort signal. It's not like Thunderbolt where DisplayPort traffic is tunneled inside a different signalling mode that the host machine supports.
This is why USB-C to HDMI adapters are unidirectional (at least, all the mainstream commodity ones). You'd need a different converter chip to do the conversion in the other direction.
I’m contemplating doing this myself at some point but my maxed out 2019 iMac upgraded to 128GB ram and extra SSD is still plenty fast for me, actually feels subjectively quicker than my M2 Pro MacBook Pro with significantly less ram feel. I was a bit surprised as I had read all the hype of the responsiveness of the Apple M-machines.
Instead, the 2018 iMac is incredibly slow, and can be thrown away.