Using older iMac as a monitor with a new iMac

The 2019 iMac is too new to run in Target Display Mode. The 24" M3 iMac is too new to use any iMac as a Target Display. So, no TDM for you. (Reference: Use your iMac as a display with target display mode - Apple Support)

Your Macs are recent enough so that if the 2019 iMac is running Monterey or better, you could make it an AirPlay Receiver for AirPlay to Mac. AirPlay was originally meant for casting photos and videos from an iPhone to a TV – and it is not as good as a hardware connection to a real monitor. I wouldn't buy a Mac just to use it as an AirPlay Receiver, but since you already have the Macs in question, it would cost nothing to experiment.

Continuity features and requirements for Apple devices - Apple Support

Stream video and audio with AirPlay - Apple Support

Aside from that, you'd be looking at a third-party workaround like Luna Display (Astropad) or Duet Display. Those also involve compromises, and with Duet Display you're looking at a "subscription" instead of a one-time purchase.

iMacs are computers, they are NOT external displays for other devices. In short, if your new M3 iMac needs an external display then you need to buy one. We get your question multiple times per day on these forums and have for years, the answer is always the same though. External displays are:

• Inexpensive (starting at <$100).
• Easy to setup.
• Dead reliable.
• Don't require expensive cabling or third party software.

In other words if you really want or need an external display for the new iMac just visit you local Best Buy, Costco, Amazon, Office Supply Store etc to find one that will suit your needs.

You can't. You would have to purchase an actual monitor.

How's that User Tip coming along?

I wouldn't know.