You may need to look at your map again - it's only about 5-6 hours all the way from Pembroke into the depths of Cornwall. So, you could break it with an overnight, but each segment would be pretty short. Most of it is on motorway, so you don't even notice the time passing.
I don't think there are any car ferries across the Bristol Channel, and even if there were you would probably spend longer waiting, boarding, crossing and getting off again than you would spend driving around.
You might well spend longer than 5-6 hours if you happen to go there on a UK bank holiday weekend as there are pretty much two main trunk roads into the area and several billion holiday makers trying to get there ahead of everyone else.
We went to Cornwall on a UK bank holiday Monday and while the road down was clear, the traffic heading back up to the rest of the country was only horrendous.
I don't think you will find too many chain hotels in that area. You will find some alright, but basing your itinerary on their locations wouldn't be a good way to do things. One of the reasons so many people go there is precisely because it hasn't become like so much of the rest of England (and Ireland for that matter) with chains everywhere. A lot of the towns are too small and quaint and charming and to be honest look like they have preservation orders on the entire streetscape, so bland corporate chain identities (which work perfectly in some locations) just wouldn't fit in well.
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