Using imac 27 inch as monitor for mac mini

Just as a reference point, Dell has a 27" 4K UHD monitor (the S2721QS) for $300. It's a pretty basic monitor – no USB-C input or wide-gamut rating. It does have an IPS panel and 99% coverage of sRGB. You'd probably spend less for it, than for a 24" 1080p monitor + all the Luna Display and Thunderbolt stuff needed to try to use that old iMac as a second display.

Review the document: Use your iMac as a display with target display mode - Apple Support

Target display mode is a feature available with late 2009-Mid 2014 iMacs. The display Mac has to be running macOS High Sierra or older and the signal producing Mac has to be running macOS Catalina (ca. 2019) or earlier.

Your setup does not qualify for several reasons.

You can't use an iMac (Retina 5K, 27-inch, Late 2014) as a Target Display Mode display, period. No matter what version of the operating system you run on it.

For 27" iMacs, the Target Display Mode feature went away permanently at the same time that the first 5K Retina Display (the one on your model) appeared.

Luna Display is $100 and your money would be better spent just buying a monitor for your Mini. Luna Display is a wireless solution that would involve all the problems of wireless video transmission. You would likely not be satisfied with the results.

Take our advice, you can't do what you want to with your old iMac

lkrupp wrote:

Luna Display is $100 and your money would be better spent just buying a monitor for your Mini. Luna Display is a wireless solution that would involve all the problems of wireless video transmission. You would likely not be satisfied with the results.

Take our advice, you can't do what you want to with your old iMac

I think you can use Luna Display with a wired Thunderbolt connection. This would take up two ports on the new Mac (one for the Thunderbolt connection, one for the USB-C Luna Display "dongle"), and one on the old Mac.

https://help.astropad.com/article/154-usb-ethernet-connection

Even with the USB-C version of Luna Display, a Thunderbolt connection, and Big Sur, you would see a maximum 5K refresh rate of 30 – 45 Hz.

https://help.astropad.com/article/226-retina-c2m

So you're talking maybe $100 for Luna Display, $50 for a TB3-to-2 adapter, and $30 for a TB1/2 cable. I believe this might make it possible to use the 5K iMac as a second display (for relatively static content). It would still be necessary to get a real hardware display (even if only a 24" one) for the Mini, because otherwise how would you set up all the Luna Display stuff?