Using 2017 imac 21'' as monitor

Only 2 options on that new of iMac...

• Luna Display does require a $79 Luna Display USB-C dongle but has no subscription cost.
• Duet Display doesn’t require any hardware but goes from $25/year for Duet Air (to get Mac to Mac, PC to Mac, and iPad to Mac support). Duet Display offers a

https://9to5mac.com/2024/02/02/use-imac-as-monitor/

Hi,

You can't use your iMac as external monitor for your MacBook Pro, since Target Display Mode doesn't support your 2017 iMac.

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

You should purchase external monitor, instead.

Third-party solution as are likely be laggy. The Luna Display support site has this:

As many modern web sites use video for even the simplest elements, your may find the lag more than annoying.

Only a free-standing exernal monitor will run smoothly and offer maximum display options. It will also use less power that a fully-running iMac, and will be easier to set up. Some uses of iMacs as externals, even with the hacks, can require cabling and adapters that cost up to US$90. You can get a pretty nice monitor that does far more for just a little exra.

Save yourself the grief and disappointment and embrace the sage advice others have offered re externals.

That iMac is too new to support Target Display Mode, and too old to be an AirPlay Receiver for AirPlay to Mac.

If you search the offerings on Amazon, for example, you can find monitors up to 27" for ~US$100 or less.

Just sayin'.