Connecting 2019 iMac (Intel) as a Display for MacBook Pro M3 Max

2019 iMacs do not support Target Display Mode. Target Display Mode went away when the first 27" 5K Retina iMac came out in Late 2014, and never returned. New Macs don't support using any iMac (even a 2009 – mid 2014 one) as a Target Display.

See the bad news here: Use your iMac as a display with target display mode - Apple Support

There are third-party "workaround" products such as Luna Display (Astropad), or Duet Display, but even if you run a cable between the MBP and the iMac, they won't be the same as a first-class hardware-supported connection to a real monitor.

AstroPad acknowledges this on their site:

AstroPad – Use Your iMac as a Monitor: A Step-by-Step Guide [2024]

"Like any product, Luna Display does have limitations and it’s not recommended for high-motion content. For example, I wouldn’t recommend using Luna Display for gaming or video editing.

Since Luna Display is squeezing a 5K display through a USB cable, when lots of motion occurs on-screen (like from video playback or moving a window quickly) the screen can temporarily become pixelated before it renders in at full resolution."

Something I just thought about – your iMac is recent enough that it can be an AirPlay Receiver for AirPlay to Mac. With AirPlay to Mac, you could use your iMac as a second screen for the M3 Max MacBook Pro – or your M3 Max MacBook Pro as a second screen for the 2019 iMac, though obviously not both at the same time.

Continuity features and requirements for Apple devices - Apple Support

AirPlay is not as good as a hardware connection to a real monitor, and I'm pretty sure that if your Mac is a 27" 5K one, AirPlay won't be able to use its full resolution.

But if you have both Macs already, for other reasons, it costs nothing to play with the feature.

+2, not a chance it will work. IF you need an external display for the MBP then you need to buy one. Why,

• Inexpensive, external displays begin at less than $100
• No expensive cabling necessary.
• Simple setup
• No maintenance required
• Reliable

Note: if you want close to the resolution your 2019 iMac has, then buy a 4K display. You can find these for about $ 600 or so.

Not possible unfortunately. Targets have long been history, buy a regular monitor.