Without going back over your design decisions, was there a reason not to use a "spindle at speed" or "spindle stalled" signal from the inverter, rather than have to create your own detection system? Sorry if I misunderstood the process.

On my large milling machine, nothing's going to stall the spindle unless I'm running a large face mill, in which case the axis drives would stall before the cutter broke. Smaller cutters go ping if they get gummed up.

That's an impressive machine you've built there, with great attention to detail!