Today I checked out a theory that a late 2020 Intel 5k 27-inch iMac could be used by an Apple Silicon Mac as an extra monitor with nothing more sophisticated that a settings fiddle and a double-ended USB-C cable. This process works as long as the Mac being used as a monitor has the T2 chip, which was a feature of the last of the Intel 5k iMacs.
Go to System Settings/General/AirDrop & Handoff then turn AirPlay Receiver on. Next, use the USB-C cable to connect Thunderbolt ports on both Macs.
Now in the Displays settings of the host Mac (the one that's using the first Mac as a monitor) and select the destination Mac as the display. This creates a prompt on the destination Mac to either accept or reject the connection request. Once accepted there's no discernible lag or delay in the video interconnection.
Disconnecting the cable drops the shared video connection.