Convert a hall-effect flow meter to operate as on-off switch
Your basic approach is correct.  This is a common requirement in a fan failure alarm.  The stream of pulses is integrated into a voltage level, and that is compared against a reference.  The more the pulses are filtered, the longer it takes to detect either the on or off states, But, more filtering makes for less ripple in the signal voltage, and this makes it possible to reduce the amount of hysteresis around the trip point.  Search for 'missing-pulse detector' and 'fan fail circuit' schematics.