2014 iMac 5k used as display

Target Display Mode went away for good when your Mac – the first 27" 5K Retina iMac – came out. Your iMac doesn't have it. Apple later added restrictions such that no recent Mac supports using any iMac as a Target Display.

There is a product called Luna Display (by Astropad) that might let you use that iMac as a second display. But even if you run a cable between the new and old Macs, there are limitations. Astropad themselves say that Luna Display has limitations, that it is not suitable for high-motion applications like video editing and gaming, and that there may be pixellation before things render at full resolution.

Astropad – Blog – How to Use Your iMac as a Monitor: A Step-by-Step Guide [2025]

Read carefully all the qualifications in this article, to understand that straightforward use of that iMac as a main display through Target Display Mode has been deprecated.

Use your iMac as a display with target display mode - Apple Support

HOWEVER:

if you choose to run some version of MacOS in that iMac, another Mac's Primary display could appear in a window on the iMac using Mac Display Sharing:

For a Mac that does not already have a Physical display, use a "HDMI Headless 4K Display Emulator, US$12.99"

https://eshop.macsales.com/item/NewerTech/adp4khead/

Then share that 'emulated' display over your network to the iMac using Mac Display Sharing.

Turn Mac screen sharing on or off - Apple Support

if this is all just "word salad", post back and readers can provide more information in any area.