I think using a scheduled event is probably the best approach.
I would precalculate the expiry datetime and store this in the Reservation entity. In the scheduled event just search for any Reservation that has gone past the expiry date time. I assume you have an association between the two entities, so retrieve the ChargingStation, increment the count and commit, then delete the Reservation.
I hope this helps. Good luck.
This is how my CheckFinishedReservations microflow looks like with two parameters, in my domain model reservation and charging station are connected