It's a real shame they got rid of target display mode. But then people would hold onto iMacs even longer.

Today, you can use software like Luna or Duet, or native screen sharing to control an iMac from another Mac (or vice versa). I have a 2019 iMac, bought for pennies and upgraded after to max out; way better value than buying the display on its own or a comparable monitor alone. The screen quality is just great.

I use it both as a daily driver and as a "thin client" for all my other machines with the software above.