2017 iMac as display for 2012 iMac

You cannot.

Read carefully the Related Article linked above your post:

Only the following older iMac models could be used as an external display when plugged into another older Mac model:

• iMac models introduced in 2011, 2012, 2013, and mid 2014 using Thunderbolt or Thunderbolt 2 cable.
• 24-inch and 27-inch iMac models introduced in 2009 and 2010 using Mini DisplayPort cable.

There is potential for you to use the 2012 iMac as a display in this way for the 2017 iMac, given the following caveats:

• The intended target iMac must be running macOS 10.13 High Sierra or earlier; it does not work with macOS 10.14 Mojave or later, or with Boot Camp and Windows.
• Retina display iMacs won’t work as target displays.
• In all cases the source Mac that you're connecting FROM must have been introduced in 2019 or earlier and have macOS 10.15 Catalina or earlier installed.

There is a software option called Luna Display

The only way I have found is a capture card or using Airplay, and yes. I am running Ventura on my 2013 iMac with OCLP and it does support airplay and it works beautifully

What OS' do you have on both machines?

The article linked above is pretty specific as to what will work.