I like the idea of port to port with a train back. If it was me id look to do something like;
Portsmouth to Poole, exploring Solent / IOW on the way and stay in Poole.
Jurassic coast round to Lyme Regis, stay there.
Lyme to Torquay,
Torquay to Plymouth including Dartmouth
Plymouth to Falmouth.
I'd be looking to do a couple of days in each leg with a B&B close to the chosen marina.
Each of the overnight options allows for a weather break out, stay longer or jump on the train.
Another option is perhaps some one on here will be heading West and could take the trailer part way / all the way, onto an easy train line (so no changes required).