Sounds fun! I would love to hear about it after, sorry I can’t help with placesin Dorset. If you came further East then Chichester Harbour offers good anchoring at the back of the Witterings. I suspect that the Beaulieu River would also be quite nice if you went West across the Solent. Hopefully others will respond with more useful info.
A number of years ago we launched from a campsite called Redcliffe Farm just outside Wareham, which is on the west side of Poole Harbour. From there the cruise creates itself, with opportunities to visit Brownsea Island, Studland, Swanage, Kimmeridge Bay, Lulworth Cove and Weymouth. There are some pretty serious headlands to negotiate too, with overfalls that would make a yacht think twice, not to mention the firing ranges, but in the right conditions it is a wonderful area to visit.