I built TargetBridge, a free and open-source alternative for using an Intel iMac as a display from an Apple Silicon Mac over Thunderbolt.

It creates a virtual display on the sender and streams it to a receiver app on the iMac. It supports mirror mode, extended desktop, multiple receivers, audio, input relay, and hardware-free receiver smoke tests.

It is a software solution, not Apple Target Display Mode, so it requires a real Thunderbolt connection and macOS Big Sur or newer on the receiver.