Another comment on peristaltic pump accuracy: those driven by DC motors, rather than stepper motors, have gear trains to provide the necessary torque. The $12 pump I found on Amazon has a maximum rotor speed of 100 RPM, but the motor is rated at 5000 RPM (probably no-load speed), so I am guessing there's a planetary gear system between the motor and the pump. If there can be a sensor on the motor shaft, the precision of the pump output can be multiplied by the gear ratio. Assuming a gear ratio of 36:1, that would make the slice thickness accuracy much better. Given the torque load of the pump rotor, I expect the motor doesn't coast very long after the power is shut off, maybe one revolution or less. Instead of a sensor on the shaft, it may be possible to detect the motor armature transitions in the motor current or voltage. Depending on the number of segments in the armature, that would provide even greater precision.