Hi,
I was thinking of using my Raspberry pi plus an accelerometer / gyro located next to the router to measure what is going on. You can get them for $4 or something from China, think it’s l2c which is available on the pi pins. Maybe too much vibration during cutting (or is it? Filter?) , but for if the Z is lowered to the surface very slowly, shouldn’t it register a bump that could be used to set the surface level height? Alternative to the tiny touch plate idea, since there is a variable height in the bits used, maybe something along these lines might work. Thought of poximity sensor, inductive ones and capacitive ones, but they are fixed height ideas or too exposed to flying debris, a tiny board like this could be located anywhere near the router really, and it could probably be a simple octoprint plugin.
You would have to be very very quiet though.
Cheers!