you could apply egress queue shaping to the high priority queues to prevent starvation of low priority queues. Egress queue shaping allows you to apply a maximum bandwidth to a priority queue, as well as a burst size. The port buffers excess traffic up to the burst size and sends the buffered traffic at the max rate, smoothing out bursts while also preventing the high priority queue from exceeding its maximum rate and starving out lower priority queues.