Using the iMacs monitor as monitor for another Mac computer (Mac mini)

Unfortunately, that setup is not supported by Apple. The iMac cannot be used as a monitor for any of the newest Macs and Target Display mode is no longer a feature of macOS, having been deprecated several years ago.

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

Per the link above:

• Your target iMac (monitor) must be using macOS High Sierra 10.13.6 or earlier. 

• 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.

However, you might consider a third-party solution, for example: Luna Display or Duet Display, though I can’t speak for the performance of those.

Do yourself a favor, avoid the headache and just budget an extra $100 for a monitor. Yes, they are that inexpensive.

This iMac is also too old to act as an AirPlay Receiver. For that, it would need to be a 2019 or later model running Monterey or later.

System requirements for Continuity features on Apple devices - Apple Support

Before you buy one fo the non-Apple options, I highly recommend researching whether Luna or Duet are laggy. I am seeing mixed reports with a slight majority leaning to "laggy". We know that Apple's network screen sharing and AirPlay to Mac (even though not available for 2017 models) are laggy.

I now believe Luna have admitted to laggginess. I just found this on their web site:

Lag means you press a key on the Mini and wait to see what happens on the iMac. That would be a minor annoyance for text but a dealbreaker for photo- or video-editing or gaming.

+1 on a proper external monitor.

You're welcome. 🙂

Enjoy your new Mac.