That's what we did, and it mostly paid off. Case in point: we took spot 103. It was a little further from the shore, and it wasn't as flat as I would have preferred (if we'd had a 6+ person tent), but compared to other spots, it had an abundance of privacy, and a network of hiking paths at your door step. Compared to our direct neighbour (101), they were directly on the road and had no privacy. Compared to spots closer to the shore, they had constant foot and vehicle traffic and very little privacy.
My tip was to check the maps (there are multiple areas to camp), select a spot at random that you fancy, and start searching that spot specifically for reviews. The net is not flooded with reviews for every individual site, but you can get a good sense of how people have fared in the past. Worked for us.
As for Sandbanks itself, there is no softer sand than those dunes anywhere I've been, it remains shallow for ages and is wonderful for children. We had an amazing time and please tell your friends it's haunted or has a sewage problem or something.
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