The DSB should be 150mm deep.
But by the looks of the sand dunes, it does look like your return pump is to big for the sump. What rating is the pump, how high must it lift the water, and how wide (front to back) is your sump at the DSB section? See the difference in the water level above the sand and the top edge of the glass divider. What is that height difference? Itlooks like way more than 10mm.
From what I learn over time, is that the water flow must be good, but not too strong to cause sand movement. Not too slow to cause excess settlement on the DSB. And that flow rate is dependant on the width of the sump.
To have the same flow rate on a wide sump compared to a narrow sump, you would need a lot stronger return pump.
I will also increase the water level in return chamber. Have the water fall down maximum 30mm. That will reduce the amount of bubbles pulled down and being sucked up by the return pump.