Download

One app on each Mac.

For your Mac with Apple silicon.
macOS 13 or later.

For your 5K iMac. Always free.
macOS 11 or later.

Then

On your Mac
Download SecondGlow Host. Open the file and drag the app into Applications. Open it, and click Allow when it asks to record the screen. That’s how your picture gets to the iMac.
On your iMac
Open secondglow.app in Safari and download SecondGlow Viewer. Drag it into Applications and open it. If it asks about the local network, click Allow.
Plug in the cable
The USB-C end goes into your Mac. The USB-A end goes into a port on the back of your iMac.
Done
Your Mac’s screen appears on the iMac. Choose Mirror or Extend in Host, like with any display.

The 7-day free trial starts the first time your iMac shows a picture. Already bought? Enter your key in Host → Help → License.