Last time I launched at Portland it was about £28 and included car parking. Really good slip. Might be worth talking to marina about berthing fro a few days to save recovering / launching every time.
Weymouth bay maybe a bit more sheltered than Lyme bay and places like Lulworth easy to reach.
If going West from West bay there's only Lyme to stop at before Exmouth - a bit of a hike, but great coastline.
Depends a bit if you want to explore or play with the toys - both fun
and what the family will tolerate