I don’t think you ever answered wether you connected the new O2 sensor to its terminals or if you’re using it for your display only.
Now, if your new working sensor is connected, I think this is what happens:
The sensor will recognize your rich mix while running chargas and will make the fuelpressure regulator trim down the fuelpressure to a minimum. When you suddenly step on it, the airflow meter opens up the fuel plunches but there is very little pressure behind them.
Now, with a sudden lean condition it will take the regulator a while to restore the fuel pressure.
You explained the symptome so well I can still remember the feeling when swithing to hybriding with the Rabbit.
Try running a perfect a/f ratio on chargas and then step on the pedal. The fuelpressure should be trimmed to fit your needs emediately.
With your homemade linkage for hybriding you don’t really need the sensor input. If you disconnect it and use it only for your display there will probably be no more hesitation.