The Rayex LEG-12F's on the MCB are miniature mechanical relays with 12V coils, and not SSR's. With the VFD disconnected from its output, I can hear the contacts clicking.
My latest thought is that I might put the "coil" of the new SSR in parallel with the coil of the spindle relay (after I identify it!) so that they actuate together, and then use my DPDT switch to selectively interrupt their
outputs. The Proxxon draws about an amp and I would not be switching it under load, and the VFD is just logic level, so my 12A DPDT center-off switch would be really safe. There would be only 4 wires running out to the switch.
Second opinion / backseat driving / sanity check welcome.
Randy