Note: If you're going to use an SBC _only_ for wake up signals, you might want to look into alternatives for the RPi such as the Radxa RockPi S [1]. My home server, for example, runs continuously at 7W, which beats many RPi models. Of course, a Pi to wake things doesn't need that much power and could be an older model, but even then, you'd still be burning "empty Watts".

Of course, the RockPi doesn't give you any KVM like functionality, though.

[1]: https://wiki.radxa.com/RockpiS

Apparently RPi Zero idles at 0.4 W: https://www.jeffgeerling.com/blogs/jeff-geerling/raspberry-p...

Yea, technically an esp32 board with ethernet would work for this job. They're typically limited to 10mbit, but if you have 10mbit/s of broadcast traffic you can afford something better.