I'll jump in as no-one else has.No doubt others who own Sinbads will then follow.I have a 27 but the principle is much the same.
The stern gear is typically protected by the upsweep of the hull from the keel area - not much clearance is required,and there may also be a skeg.The Sinbad hull is described as having additional GRP layers and reinforcement to allow routine drying out.In my experience the hull will settle happily in soft mud and in effect will float in it so that the stern gear lies clear.There is sometimes concern that mud/sand particles will enter the cutlass bearing and accelerate wear,depending on you mooring situation.
But whether hard or soft the main concern is whether projecting debris on the seabed will provide a point load on the settling hull.Shingle areas which displace to the form of the hull are normally OK for short term anchoring and are illustrated in the original brochure.Drying moorings are normally placed in clear areas of bed.My manual suggests walking the bed around a temporary anchorage just in case there is the odd half brick present.Obviously you have to time this and mistrust over-soft areas.
Small bits of debris will gradually sink into soft mud and would be driven down as the hull settles anyway.
From reading it sounds as though the Sinbad will sit on it's raised keel to give a little more clearance on a hard bed.
If this helps.
Peter