@Werner I imagine there are plenty of people here who could help solve similar problems. In theory, I could provide a pair of an old RPI (master) and a 4A (slave), with the slave serial connected to the master and power controlled via the GPIO, and they are also on the same VLAN. However, if something goes wrong it needs control to the slave's SD card, all I've found is an SD card extender, and it needs something like an emulator that connects to the master via the same USB/SPI.