there are some pain points during the day, particularly with regard to
departing heavies one after the other after the other
But that's actually what you want; you aim to group Heavies together as you do not have to apply any wake turbulence separation between them.
Theoretically, mixed mode does cut down holding, both on the ground and in the air, providing the schedule is the same; both the departure and arrival rates are in excess of the schedule. So, if you keep running mixed mode for a period of time, you'll eventually run out of aeroplanes and they'll just land or depart as and when they turn up.