I have had the same issues with finding 4mm (possibly 4.1mm) barrel connectors. I’ve resorted to cutting the power cables of the supplied mains adaptors.
Wikipedia documents many, many coaxial power connectors. I think the GL-X750 must use an “EIAJ-02”, which is conventionally used for 3.1V-6.3V supplies. My guess is that was chosen as a 5.5mm socket would have been too big.
GL.inet and others seem to be moving to USB-C for power levels above 8-10W.
It’s possible to supply regulated power - 5V or 12V - directly to header pins on a Raspberry Pi, Jetson Nano, etc. I suspect that’s possible with GL.inet devices but I haven’t seen it documented anywhere.
Cheers
Sean