The decision was made to require the printer to be on the edge of the desk & let the string hang.  There's enough clearance from the bed movement for an animal to get in & change filament without moving the printer.

A simple structure routed the string.  Detecting the runout ended up requiring just a simple lever.  The string system ended up vastly simpler than anything else.

The switch was an instant fail.  Not enough pressure to reliably press it, so in went the photo interrupter.